Love Your Mother. It would work just as well for mom's birthday as for Mother's Day.
(A brief digression: Speaking from personal experience, if you're thinking of getting a tattoo of any kind, chances are, your mom won't be pleased. It doesn't matter how young or old you are. You are still her baby and you shouldn't go messing around with your "perfect" skin - which, incidentally, she thinks of as hers anyway.)
Despite the fact that it's cold December and only a few weeks from Christmas, you can pick up this mom-centric card over at Empty on the Inside. If your head is wrapped up in tinsel, snow, pine trees, baking cookies, shopping-days-left and blinking lights, there are plenty of unique, hand-painted holiday-themed cards to be found as well.