This is the controller I use. Sadly this controller is unavailable now. But you can just look up PDP Xbox controller and there are several other colors and designs. and they're decently priced.
Buy this, it’s awesome!
i m looking at this one rn
Repair Kit for xbox one/360/X PS4 PS3 PS5, JOREST 25pcs kit with PH0 and T6 T8 T10 Torx Security Screwdriver, Crowbars, Tweezers, Brush, Grip Caps, Screws, Cleaning Tool for Controller and Console https://www.amazon.com/dp/B08TWV3BXB/ref=cm_sw_r_cp_api_glc_fabc_QV1A1MGSBJAC9SF4W7QY?_encoding=UTF8&psc=1
There are several of these that work. This is the one I used.
Or even better because they're thicker, use a playing card mat, something like this they're less expensive and again thicker.
Same thing happened to me in the same stick, same controller, guess it has something to do with red paint interfering with the rubber compound. Anyway, keep the broken cap because you'll probably have to put a new one over it. I used these ones https://www.amazon.es/YoRHa-Profesional-Thumbstick-pulgares-Controller/dp/B07559MMRW/ref=sr_1_25?__mk_es_ES=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=xbox+one+stick+caps&qid=1625131959&sr=8-25 but you can find cheaper ones.
Honestly if the actual stick(the piece that still in the controller) still works you could probably buy a new one thumb stick off of Amazon to replace the broken piece Kind of like this (https://www.amazon.ca/eXtremeRate-Multi-color-Thumbsticks-Replacement-Controller/dp/B01LPF54GI/ref=mp_s_a_1_3?dchild=1&keywords=xbox+one+controller+thumbstick&qid=1611947720&sprefix=xbox+one+controller+th&sr=8-3)