I've gotten some assets for polygon mumbai here: https://faucet.paradigm.xyz/
Should be able to trade those for USDC/USDT. From a quick search, can trade for USDT on Mumbai here:
I tested it, and was able to swap MATIC & wMATIC for USDT.
I tried adding a little liquidity to / creating a (mumbai) MATIC <-> USDC pool for you. You have to put in the Matic USDC token manually to see that pair, that token address is here: 0xe11A86849d99F524cAC3E7A0Ec1241828e332C62
(There's only 5 MATIC/5USDC in that pool, so only suitable for small amounts.)
This faucet also gives out USDC directly: https://calibration-faucet.filswan.com/#/dashboard