Good luck. This is a legacy product category. The cheap units that only last a few months may be about the same cost as renting a DVR box from the cable/satellite company. But it's only a matter of time before even those cheap VCRs cease to be available. Unless you think your mother has little time left, you need a strategy to move her to newer technology.
Assess her usage. Does she record shows, watch them, then record over them? Then getting her a DVR, especially if you can get it as part of the cable box for a minor cost, and teaching her to use it makes more sense than hunting down VCRs for years to come. Yes, it can be difficult getting old people to understand the technology but just treat it as a VCR with no tape to change. She may not get the full use of the tech but as long as it fulfills the purpose, who cares?
If she goes for long term retention of recordings, then you have a more difficult situation. A DVD Recorder can be pretty effective and the nicer ones have hard drives so that several shows can be saved and arranged to disc as desired. This means some learning curve but you're going to have a lot less trouble keeping her supplied going forward.
Alternately, if she likes to keep favorite shows long term, download them and burn them to disc for her. There are DiVx capable DVD decks that start as low as $40 that will let you put a lot of stuff on a single disc. I've done this for my own mother. Even if I'm able to get her the box set of the show wants, it's still easier to have it on one or two discs than half a dozen or more.