you want to buy ipb skins? If so then here is a good one http://www.transversestyles.com/index.php?tsv=products
you can get some skins for IPB at http://www.invisionize.com. All the other ones you have to pay for.
Invisionize really isnt that good. Its a good source for mods. But for skins i wouldnt suggest it. Invisoneers is good. Also check out Get Skinned. http://www.getskinned.co.uk/forum/