Wodfitters, Draper’s, Serious Steel, Rubberbanditz, and Rogue Fitness would be some of the most popular brands of pull up assist resistance bands in my opinion. They can be found on Amazon, or directly on their own respective websites. Just make sure you get a set with at least 4 bands.
As for the rings/handles varieties, that’s entirely your call. Do you want your set up to mimic more dumbbells or a barbell?
If you do decide to go with the ring/handle variety: Wodfitters or Garage Fit are a great choice.

Here are some links to get you moving in the right direction:

WODFitters Stretch Resistance Pull Up Assist Band with eGuide, 4 Band Set Special https://www.amazon.com/dp/B01M0XL1H8/ref=cm_sw_r_cp_api_i_ZcoZEb3FSM40W

Garage Fit Pull Up Assist Band with Metal Handle and Carry Bag, Stretch Resistance Mobility Band -Extra Durable, Cross Training Exercise (Bundle1- #0 Yellow, 1 Red, 2 Black, 3 Purple, 4 Green) https://www.amazon.com/dp/B0851NXXV2/ref=cm_sw_r_cp_api_i_HwoZEbSK8F5NF

EDIT: FORGOT TO MENTION THE ELEVTAB BAR IS BACK IN STOCK RIGHT NOW AND IS SIGNIFICANTLY CHEAPER AND COMES WITH 2 BANDS!
https://www.amazon.com/dp/B085VKWFNW/ref=dp_prsubs_1