Make sure you get T posts and not the cheaper U posts, they're weak and will bend. A 6' T post here is just a few cents more than a 5 1/2" post. A 6' post driven in till the foot plate is in the ground will give you 4' high over a single pallet on the ground, so I use them rather than saving a few pennies on shorter ones.
Most of mine come from farm auctions though, whenever I get low, I'll send some money with a friend who goes to a lot of auctions and usually get em for half or less than new ones.
I drive em in straight up, they'll lean out just a bit when loaded, but I've never had one give out on me.
A fence post pounder (looks like a 3' piece of pipe with one end capped and a pair of handles on the side) is a tool you want if you need to pound more than just a couple posts. Fairly cheap at about $20 around here.