MTron Pro from GForce has been around a very long time. Not cheap.

IKM Sample Tron is fine, but it's 32 bit. You didn't say what version and system you're using, so that may or may not work for you. You should be able to get it cheap right now. IKM will be going 64 bit in July and you may be able to upgrade.

If you google, you'll find all sorts of free Mellotron sounds people have sampled from their instruments and uploaded. You can load these into most sample players and get a reasonable facsimile. Mellotron sounds are pretty low rent anyway, so these should work fine.
I followed the instructions exactly, but when the installer opened the Sampler Instruments window, I dragged all four unzipped folders into it. There is something wrong with the folder names, and it looks like the one that has the instrument in it is not in the one folder you are supposed to put there. Putting all four in there works, although I'm not sure why.