I am replacing a shower head in a shower stall with a handheld shower head. The bracket that holds the handheld shower head when not in use will be positioned too high for my wife to reach it. Do they sell a threaded piece of pipe (maybe 3-in long) to extend the pipe from the wall thereby lowering the shower head’s resting position just enough so she can reach it? Otherwise, are there other options?
How about buying this?
https://lh6.googleusercontent.com/-wRZjPGJvYGk/VEyQ4sdhY2I/AAAAAAAAYDk/m1GgYU_Sjfg/w140-h220-no/shower.jpg
Or one of these
Yes. Take your existing fixture to a plumbing supply store in order to match the length desired and thread size.
Yes. Any plumbing supply store, or even Home Depot, will be able to cut and thread a piece of pipe to suit your needs.
Underline mine
Your Wife can’t reach the handheld shower head? The one that is mounted on a flexible hose mounted to the wall of the shower?
Or do you want to lower the main shower head?
Probably both! Many of these hand held shower heads thread onto the pipe that sticks out of the wall. They replace the original shower head. The shower is on one end of a hose and the other end is what threads onto this pipe, lets call this end an adapter. When the hand held shower head is not in use, it clips onto this adapter that threads onto the pipe in the wall. So for example, if I installed one of these in my master bath shower, My 4’ 10" would have a hard time reaching the shower head which would be 6’ 10" above the ground. She could touch it, but she probably could not un-clip it. I would have to use a 9" extension between this pipe in the wall, and the new hand held shower head adapter. Clear as mud?
As for the OPs question, you can find sort lengths of pipe in your hardware store. You will need a coupler (double female) on one end of this pipe to use it as you plan to. No problems, they are in a bin near the short pieces of pipe. Be sure to get the correct size of pipe. They come in 1/2", 3/4", & 1" inside diameters.
IHTH, 48.
Cute, Bob! 
To clarify: At first, I was thinking she can’t reach the handheld shower head mounted in the bracket. Now, I realize she won’t be able to reach the diverter, either. I visited Lowe’s and Home Depot. Why do they sell shower pipe that has male threads on both ends? Of what good is that? In general, it seems there is no standard pipe. The next best thing is to leave the diverter in one position, and let the shower head hang down, or buy a bracket that suction-cups to the wall. You can’t tell me no other woman has run into this. The shower head assembly is basically useless without being versatile. 
If the osteoarthritis in my shoulders get any worse I may start to have a problem with this. I just have to keep up with my stretching exercises. I can grab the hose below the holder and push up to get it out, but that makes it flop around a bit until I get a better hold on it. I like the idea of you trying to mount it lower for her; I hope you find a good way to do it and let us know how it turns out.
Are you looking for one of these? They’re “shower arm extenders,” and allow you to reposition the shower head farther up, down, or to the side. (Here’s a picture of one installed so you can envision how it works.)
The pipe in the wall ends with one of these, and you should already know what threads a shower head has. The part you’re looking for is usually called a shower nipple.
CMC fnord!
Its called a nipple, and it is universal in pipe fitting. The simple reason is the pipe is a cylinder; it is not bigger on one end. If you put a female thread on one end it is going to have to mate with a completely different size pipe. The functional reason is you connect it all with female threaded fittings. The female fittings come in fixed sizes, a pipe fitter cuts pipe to the size they want, threads both ends with a threader and off they go. for your purposes the shower head and shower fitting in the wall are designed to be connected with a shower nipple as pictured in a post above.
Take the old too-short nipple out by simply unscrewing it from the wall and install a larger one; or get a elbow or coupler and extend it (not pretty). You need thread compound or tape on the threads mating the fittings, the shower head will have a washer.
Hardware or plumbing stores will often have a selection of shower parts including extension arms, diverters, wand brackets and height adjustable tracks for wands. Your best bet is to piece together a working solution from these, or just buy a shower wand with a height adjustable mount and diverter.
There are also accessories that glue/suction to the wall as extra brackets for a hand-held shower unit. Some people like to put them at stomach-height either to spray their belly or to be able to sit in the bath while rinsing the shampoo out of their hair.
Look at Bed/Bath/Beyond or similar places for a universal bracket. Or contact the manufacturer of your shower-head for one that fits the handle properly.
–G!
The handicap shower stall at my gym employs one of these for their hand-held shower head: Moen 3868 Chrome Single Function Hand Shower Package with Hose and Slide Bar Included - FaucetDirect.com
The hose screws into the shower head up high and then clips into a slider on a bar on the wall. You can either position the sprayer and clamp it at the height you need (presumably while sitting down) or just take the sprayer off the slider bar and use it in your hand.
Check out options for accessible shower fixtures instead of just for people who merely want to hold their shower head in their hands.