I believe i have a hamstring sprain, behind my knee, which i have been treating with rice. however, knee stability is weak. knee brace good idea?

Symptomatic relief: A brace may help relieve your symptoms, but if your knee continues to be unstable you may need an MRI to check for ligament damage or other problems.

Not necessary: Hamstring strain is muscular and generally heals with time. Ligament injury about the knee is what causes instability which might respond better to use of brace.

Many best braces: Each brace is indicated for a different purpose. Each has their strengths and limitations. You will need to be evaluated by a professional to get the best brace for your condition.