@oleaster I use pull up assist bands,
like these. They aren’t made for this purpose; I just triple-loop one around my knees. I like these, instead of the dedicated, short, broad booty bands, because they’re multi-purpose—pull ups, dips, and warm ups.
The brand I bought was UPOWEX, through an Amazon lightning deal, and I don’t see them available right now. Bands have very much become a commodity item. Do check the reviews, to be sure the production quality is good, but otherwise, there isn’t much to differentiate them.