i would only add 1 personally. they will end up getting big eventually as well. i have 1 conch in my 58g tank right now and it has been happy from day 1. although i had a total of 3 at one point in time wich eventually all ended up vanishing besides the one. as far as a good place to buy one, there are alot of stores that sell them so i guess find who has the best deal and buy there. i bought mine from saltwaterfish and that fish place.