In your situation, check out Tirerack.com
It might be cheaper for you to order your tires on-line and have them delivered versus buying them in your area.
I paid roughly $130-145 per tire for the LT245/75 & LT265/75 16's = 2 different trucks, installed from Firestone. Could have gotten them on-line, but for 16" size the overall price and having them delivered, installed would've been the same for me, if not more.
However, 15" tires are quite a bit cheaper - depending on the whole overall size, of course. THere's times I'd love to have 15" just for cost purposes.