There are TONS of products out there now-a-days to help with arthritis. Even some foods (usually large breed &/or senior foods) have supplements mixed in for arthritis/joint problems.
The most common ones are Glucosamine, Chondrotin. Bovine Cartilage is another fairly comon one.

Your best bet would be to speak with your vet if you are just starting off starting the supplements. They can tell you what type would would be better (depending on where & what form of arthritis she has) and the proper dosage.