Many people interested in Celtic languages have taken advantage of the various Internet referral programs and set up links to books on theirsites. If you are click on a link to Amazon from another site and then buy something, chances are someone is receiving a percentage, which is even higher if the link was to a specific book. Other referral programs are very similar. If you are interested in finding materials without using the Internet, it is certainly possible. You can look for Celtic books almost anywhere, but you'll have trouble finding them unless you locate a shop specializing in things Celtic.
Specialists are naturally the best.
If you are on the search and happen to walk by a new age shop, it might be worth checking, because sometimes such interests coincide. But if you browse the section of your nearby Borders or other corporate bookstore, you're bound to found a few books in Celtic languages. It's better than most smaller local bookshops, but the selection is still very limited. Not all smaller shops may do this, but the corporate stores always do. This is different from the Celtic specialists, such as those online, because they actually stock most of the books.