Sure, you could spend some time wandering around the store, hoping to find just the right card for Mom when Mother's Day rolls around. Or you could make things a lot simpler by printing your own card, and customizing it yourself. Here's a list of customizable and printable Mother's Day cards.
The DLTK site has a lot of printable card resources, from poetry cards for Mom to these wonderful four-fold cards that will remind some of you of the fold-out on the last page of MAD Magazine. The templates are downloadable and easy to use.
If you want to make your own card but coming up with the right sentimental thoughts is not your strong point, take advantage of the work done by the folks at American Greeting, who have put together exactly what you want to say but couldn't quite put into words. Only a few of the templates are free, but if you're going to spend money on a card anyway, it might make sense to use one of their designs and poems and then customize and print it yourself.
Denise Witmer, About.com Guide to Parenting & Family, has a half-dozen Mother's Day cards with lovely images on them that you can print yourself. The cards are in PDF format, which means that you can't edit them easily--but the pictures are great and all you have to do is print and fold the cards, and add your own comments by hand.