I obtain Linux ISO's by purchasing "live" DVD's and using various burner-programs to generate ISO's from the DVD's. Before using any ISO, calculate its checksum and compare it to the corresponding reference value on the official website, or on Distrowatch. It is a very bad idea to use an ISO which has not been validated in this manner, because some of them contain dirty tricks which can lead to loss of valuable data.
All of the Linux DVD's which I've obtained from Ebay seller ZC Trading, including Kubuntu 20.04 and Ubuntu Mate 20.04 (which were the latest at the time, except for the daily releases, which I don't trust), have arrived quickly in good condition, and have produced ISO's with good check-sums. If he doesn't advertise what you want, contact him.
To calculate a checksum, I recommend placing the ISO of interest on an hdd or ssd on a PC with some power. Then right-click in the directory where the ISO is located, and on the menu which appears, select the option to open a terminal in that directory, and enter "sha256sum <filename.iso>," without the quote marks and with the ISO's actual filename. To get the actual filename, right-click on it and select Rename, then press Ctrl-a, then Ctrl-c. Then paste the filename into the command by pressing Ctrl-Shift-v and hit Enter. I haven't timed it, but it seems to take about half a minute on my PC to perform the calculation. If you want to copy the result, highlight it and press Ctrl-Shift-c, but if the checksums aren't identical, the differences won't be subtle.
