Jay, if you can find some newer ones that are the length you want that would be ok. Definitely don't get older "telephone" poles and then cut them. There are chemicals in them they you don't want in your yard or around your little ones. It's why no one at Georgia Power will be giving away old poles, too much liability.
If I was you, the safest thing would be to do the 6x6 PT at Home Depot.