Open-source usb TRNG and US standards - Help choosing
If you fish around in your pocket, you can probably find one that costs 0.25 USD.

All it takes to use one of these gizmos is a little patience, a bit of hand-eye coordination, and some confidence that there aren't any surveillance cameras watching you, which you need anyway because you just typed in your password under them to decrypt your laptop's disk.

Once you've warmed it up by flipping it 256 times—or 512 times, just to be on the paranoid side, in case the entropy is ever so slightly below 1 bit per flip—you can get gigabits per second by using the outcome as the seed for an AES-CTR or ChaCha stream.

(You could even drive the cost down to 0.01 USD if you have the extra dexterity, but when the 0.25 USD option will still leave room in your budget for three or four nice dinners including booze, I figure the extra trouble of flipping a penny instead of a quarter isn't worth the 0.24 USD.)