Actually, it’s the 20 years or so I’ve worked in lumber yards, including counter sales. 
Has actually looking to see when a message was posted gone completely out of style? Oh yeah, it’s the Dope, where reading for comprehension is optional.
Do they sell fence posts that are 10’ long?
Not usually, but you may be able to find treated rough cut 4 x 4’s 10’ or 12’ long. I would recommend calling a local lumber yard. If there is local demand for a product, they will usually have it. Big box stores usually won’t have that type of item.
That would work, except that the point of these kits is supposed to be that you just drop the 4 x 4s into the nifty metal connectors and bob’s yer uncle–instant hammock. You’re not supposed to have to futz around with glue and stuff.
I would think that a rough cedar fence post, even if you could find five of them the right lengths (the kit requires one 8 ft long, two 6 ft long, and two 4 ft long), would require a lot more sanding to make them safely splinter-free than off-the-shelf lumber.
Lowe’s cedar fenceposts only come with holes punched in them, to receive the rails, and they only come in 64" lengths. If she was going to use cedar fenceposts, the OP would have to go to an actual lumberyard and bespeak five custom-cut posts. Here’s a lumberyard that offers 4" fenceposts in 6-1/2, 8, and 10 foot lengths. So (assuming that she lived in Timpson, Texas) she’d need to have them custom-cut four of her five fenceposts.
Again, the point of the kit is supposed to be that you just zip down to Lowe’s, load five 4 x 4s into the back of the minivan, take them home, plop them into the metal widgets, and relax in your new hammock.
It ought even to be possible to find planed fence posts that are true 4 x 4 - I’m pretty sure I’ve seen them (but that’s here in the UK of course) - they’re usually pressure treated with a copper based wood preservative and are intended for fences more solid than the ubiquitous post and panel jobs. Actually, they might be sold as gate posts.
Posts with holes in them are for rail fence. The posts she needs are for privacy fence, and are typically available in 6’ and 8’ lengths. No special cuts are needed. Four footers can be obtained by cutting an 8’ in half.
…which she would still have to pay the lumberyard/Lowe’s to do… 
Related question. I know nothing about lumber. When I was a little kid, I remember my dad would sometimes go to the hardware store and buy lumber he referred to as “4 by 4” but to me it looked quite rectangular. I would have expected a square prism about for inches by four inches, but this was not what he was buying.
Or else I’m remembering things wrong. But I remember feeling very confused about the naming conventions.
So riddle me this. Should a “4x4” be a square prism or a non-square rectangular prism?
-FrL-
I am a bit unclear on what you mean by “prism,” but the cross section of a 4X4 is a square.
Usually, most places give one cut per board free. I don’t know about Lowe’s in particular.
Or cut them herself, if she feels comfortable with it and has a circular saw.
The first cut at Lowes is free. The second is, I think, a quarter. Maybe fifty cents. Hardly a deal breaker.
Just FTR, and not that it particularly matters at this point, but I would have had to have gone with a 10 foot board rather than 8, since the hammock was an XXL Mayan.
They’re not oddball. They prefer “plain”.
I’d go with the box of thin wooden wedges they have for sale within 20 yards of the 4x4’s.
Cartooniverse