I do. You can find it occasionally on eBay. The easiest thing to do is to set up a search and follow it so it emails you whenever there is a match. “mew cassette tape -pokemon” or similar should work.

musicstack.com is a great place for this!

Also discogs.com. I filled up most of the rarest parts of my collection from there.

What do you need – I may have a few bits and bobs depending on what’s missing..?