Some materials may be available at your local bookstore, or they may be able to order them. But many need to be mail ordered. There are other places to look, too. I have included those which specialize in Gaelic materials, or which seem to have stocks of them consistently. For music sources, you might also do some web searching for the record companies.
Many companies have a web presence.
I recommend some comparison shopping. But, remember that US or Canadian resellers, although they may get a discount for volume, will also have to pay postage, and will need to make a profit. In short, you may or may not be better off ordering from a closer source. Remember too, that speed and good service may be more important than price in some respects. If you find a source you like, it probably pays to stick with them, even for the occasional more expensive item.