If you want something larger than the Lightweights Power Reflectors pieces that I used on my gloves, they also sell it by the roll,
Lightweights Flex Tape
It's an industrial-grade 3M reflective tape sliced down to consumer-size rolls.
For larger reflective patches on jerseys where you want to maintain breathability, try 3M 5510 "slash" pattern heat transfer at
Identi-Tape.