I bought all my live rock through ebay... though it's like a box of chocolates... you never know what you're gonna get.
I paid $149 for a 49 Lb. box of Fiji, and $129 for a 25 Lb. box of Tonga.
With a 38g tank, you may want to rethink getting a tang. They need lots of swimming room, so the usual minimum tank size for tangs is around 100g (though you can keep yellows in a 70).
While you 'can' keep them in smaller tanks, they'll be constantly stressed and will become ich magnets. I have a friend keeping a powder blue tang in his 65g reef (it's about 2.5" long) and he was befuddled as to why it kept getting ich, until I explained it to him.
HTH