There are a surprising number of places you can check for Latvian learning aids.
I'll be updating this frequently over the next few weeks as I add in all of my links and books.
I strongly recommend Jānis Roze. They will ship anywhere for close to the actual cost charged by Latvijas Pasts and have an absolutely stellar selection. If a book is really expensive here in the US, chances are you're paying a lot to those middlemen to import it.
Also, Zvaigzne ABC, a Latvian publisher, is now offering a small collection of ebooks through their Grāmatu Klubs (book club). The books seem to be only offered in Adobe Digital Editions format, many readers support this format, but check yours before buying.
Baltic Shop offers all sorts of merchandise from the Baltic countries. They have quite a few books, but I would check Jānis Roze and calculate shipping/currency conversion charges before buying.