good luck actually finding it at a walmart, your best bet is to order from amazon or walmart online, I paid 10$ for mine and it was worth it, the mount alone is worth most of that. as to buying diamonds, pick up a card and just hang onto it, diamond sales happen often and its only worth it to get diamonds when they are on sale, and as TheDarkSage said, hang onto the diamonds till past lvl 50, usually i say 60 now but 50 at least, then ask whats the best to buy, tho the answer is going to be purified fusion stones,and other things that make your gear better. Hope this helps.
Jinkz
Govinda