Since any nominated card must have an available miniature to go with it you can check the availability of your miniature using the links below to various online miniatures games stores.
Site Sponsors:
Auggie's
Cool Stuff, Inc.
Miniature Market
Troll and Toad
Wargamer's Headquarters
(House Mouse Games is a Site Sponsor as well. The above list represents the Site Sponsors who carry other games in addition to Heroscape)
Other Stores:
Fantization (one of my favorites)
Games Empire
FRP Games (another really good one)
RPG Shop This one seems to have disappeared
Noble Knight Games
Paizo
The Warstore
Reaper Mini
Crocodile Games
Privateer Press (Hordes, Warmachine, Monsterpocalypse, Voltron [if only Voltron was the right scale])
Role 4 Initiative (D&D)
Wyrd Miniatures (Malifaux)
Games Workshop (I know, I know, but some people do use their sculpts)
Icon USA
Team Covenant (they specialize in Monster Apocalypse)
There are others but if a miniature used for a custom can't be found on any of these sites, chances are it is no longer available.
Also, sites like Cool Mini or Not probably wouldn't be acceptable as a source for availability. It has a gallery of some incredible paint jobs (visit it if you've never been there) but it isn't a good source to purchase from.
If you know of other stores that offer something in miniatures that you don't think any of the above offer feel free to let us know through PM and it can be added to this post.