Pilates Resistance Bands What Are The Bands Called That Are Used For Pull Up Assists?

What are the bands called that are used for pull up assists? - pilates resistance bands

I'm trying to improve my flexible arm, but I have the resistance bands can hang on a bar set aside. they look like giant rubber bands .... If you do resistance bands or something that I used those fragile plastic in Pilates. What these groups are called and where can I buy (online or not)