Where would one look for tooling, if he were to consider his hand at checkering?
Start at Brownell's. If it's available at all, they probably have it. Check out Amazon too. Ebay.
Lee valley carries some of the fine veiner chisels, useful for cleaning up a line end.
After that, they pretty much come from where you can find them at.
If you can round up a copy of Gunstock Checkering and Carving, by Kennedy, he shows a lot of pictures of user made tools, most just a bit of file work, then hardened. Usually they use a Swiss Checkering file (as used for filing the checkering in to a hammer or across a section of frame for grip), to cut the teeth.
I have used three and four square needle files, sometimes modified, sometimes not, to chase a line out too, but it would suck to try to do a checkering job entire. The corners are not always the sharpest, but you can grind off or use a diamond file to remove the teeth on one edge, and make it clean up pretty well.
Gonna depend a lot on your own levels of Craft too. You can make the tools for cheap, but that costs you time and effort. Or you can spend a fair bit and skip the preliminaries.