The common brace for patellar (kneecap) instability is the 'do-nut' type, with a cut-out for the patella, and firm support to either side. There are also 'strap' braces which go around the top of the shin over the patellar tendon - for patellar tendinitis.
Neoprene Knee Support with Stabilised Patella - For general knee support and mild patellar instability
FormFit Knee Brace with Hinged lateral J - For more serious patellar instability
Cho Pat strap - for painful tendinitis of the patellar tendon under the kneecap