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What kind of painting? Nice brushes are expensive but last a long time if taken care of, but they are dependent on the type of painting (oil/acrylic/watercolor). What about a gift card to AC Moore or Pearl Art or whatever art store she has nearby? You could package it up with a nice sketch pad and a set of nice drawing pencils to make it seem a little more personal.

ETA: I did oil painting in college but haven't done it in years, so I'm thinking of what I'd want if I was going to pick it back up.

Hmm... does she work on canvases? You could get a couple of size canvases (assuming she doesn't usually stretch her own), and a starter set of paints. Depending how much you want to spend, if she is working with acrylics, something like this would be awesome:
http://www.utrechtart.com/Utrecht-Artists--Acrylic-Paint-Set--Deluxe-Wood-Box-Kit-MP-01653-006-i1002444.utrecht

Or a little less $$$, this is a good base set of colors to start: http://www.utrechtart.com/Utrecht-Artists--Acrylic-Paint-Set--Compete-Kit-10-Colors-MP-01653-005-i1002443.utrecht

There are different size tubes and sets, depending on your budget, but I think a nice gift would be a set of paints, maybe some canvases, if she uses them, and brushes-- the brushes can get very expensive, though. When I was in school I always found it helpful to go to an actual art store and the workers could usually point me to a decent quality brand that wasn't crazy expensive. Are you able to go in person and bring it to your mom in the middle of nowhere?

@Weagle: Honestly, going to a nice art store and walking around and picking stuff out is really fun, especially if it's already paid for!

I like dickblick and jerry's artarama for online purchasing. And canvases are always welcome like @Danizaur mentioned.

Does she have an easel?

My mom is an artist and orders a lot from dickblick, on the whole their products are a lot better quality than what you'll find at craft stores like AC Moore. I know dickblick occasionally does groupons too!