You want something like this: https://www.fringesport.com/collections/strength-bands/products/latex-free-strength-bands

Doesn't the matter what brand that's just the first thing I came across to try and find a product for it. Nice and wide so it can sit comfortably and provide a decent amount of resistance. Double them up if you need more help.

One way to get started without the bands is to just jump up as high as you can and try to grab onto the bars at your highest point, then try to lower yourself back to the ground as slowly as possible.

Is your goal getting good at pull ups, or is strength or hypertrophy the goal?

I have this set and they're great https://www.elitefts.com/elitefts-single-band-pack.html

I use them for regressing a bunch of different bodyweight exercises like pullups and dips. Like another poster mentioned, you should also do negatives because the bands provide the most help at the bottom when your muscles are fully extended, which is often their weakest point.