Try Miniature Market, Strikezone online, and ebay. Several of the ebay sellers deal in bulk so they have pretty good prices. Personally I like Woolverine (that is correct spelling) and this site
clix the gathering
EDIT: Miniature Market has the Dark Knight figure (Search "Heroclix Dark Knight")for 50 cents, but hurry! They are running out and this deal will only be available to the next 433 customers!
DISCLAIMER: C3G claims no ownership of the characters or artwork used for C3G customs. All rights for the characters belong to their respective publishers/creators. C3G cards are not intended for sale, and C3G does not authorize any party to profit from C3G cards.
RIP George Perez and Stan Lee