www.tirerack.com

You can just put in the year, make, and model of you mom's car in tirerack's website and it will give you a list of all the tires that will work with her car. They also have user reviews of each tire on the site. Tirerack's got a pretty good selection and competetive pricing. If you prefer to get your tires locally, ask the shop to match their pricing. Good luck.