Found these at walmart.
Got the tumbler for my FIL and he enjoys it, says it's a great size.
https://www.walmart.com/ip/Ozark-Trail-16-Ounce-Durable-BPA-free-Silicone-Collapsible-Travel-Cup/49759390
Also found this looking up collapsible camping cups. Looks like a decent cup and has a hole for a tether or carabiner.
https://www.amazon.com/UCO-Collapsible-Backpacking-Camping-Venture/dp/B0849HT3VN/ref=mp_s_a_1_4?dchild=1&keywords=collapsible%2Bcamping%2Bcup&qid=1609037165&sr=8-4&th=1&psc=1