Unofficial answer to Rubber Band Ruling

There are two allowable sizes. #32 or #64. Any other size is illegal.

Packs are generally sold as that size - at least in the US. You can’t go wrong with Alliance rubber bands (the non latex ones are the ones Vex sells and they are very consistent)

#64 Alliance rubber bands with prime shipping so you will get them in 2 days.

Size 32.