Suppose I wanted to use my vintage camera for flash photography. Do they still make (5B, in particular) flash bulbs? Or are they only available from eBay and vintage places?
Try this link.
There’s one outfit in Ireland still making new ones, but they’re really big ones, and shipping is prohibitive.
Flashbulbs.com is a real ripoff. The #5 (and Press 25; same thing, different brand) was the most common size, made well into the '70s. You can get them for $5 a dozen on eBay if you look hard enough. The usual eBay price seems to be a little under $10 a dozen.
The B after the number means they have a blue coating for daylight color film; if you’re shooting black and white, use clear ones – they’re cheaper. They were also available with amber coating for tungsten film. Any color bulb is fine with BW, obviously.
You can also get an adapter to use the tiny little AG-series bulbs (most commonly used by Polaroid cameras), which were the last gasp of the flashbulb industry. They put out almost as much light as the #5, but are quite a bit smaller and a whole lot cheaper/more common (of course, with the adapter you can’t use the eject button on your flashgun, and have to pull out the used ones with your fingers).
My friend and I got some at a government surplus sale, which means that ebay may your best choice.
Unfortunately we didn’t pick up that many, and the ones we did had their bulbs wired up together and set off at the same time.
BTW, even with a piece of paper over them, 8 flashbulbs are gonna leave you blinded for quite a while. 
i have an industrial application that requires testing using an ag1 flashbulb. they are getting harder to find. i am not sure what we will need to put into the procedure to replace this test when they are no longer available