Archive.org has a lot of 80's and 90's cartoon themes in its database. The downloads are free unless otherwise dictated, and they even have the extended version of the theme from M.A.S.K in there.
https://archive.org/index.php
I would run a security check before downloading files, though. Some people who have taken magazines from the "Men's Interest" section of the website have been treated to viruses.
This website
http://www.televisiontunes.com/ also has cartoon themes like you wouldn't believe! I mean you can get lost in here for hours listening to everything in there. They have some of the most obscure, "I can't believe they have this here" Television themes. For instance, they have the theme to Charlie's Angels pilot in there! (which is different than the other season by season themes)
This website has anime themes (And I mean every Sailor Moon show, movie you can think of) Power Rangers (including the original Japanese Sentai themes like Battle Fever J, Goggle V, Kyoryu Sentai Zyuranger just a drop in the bucket Kamen Rider too)
They've got music from Jem and the Holograms mastered from the cassettes that came with the dolls, G-1 Transformers themes and information files that aired at the end of the episodes, G.I. Joe ARAH music. There's also the ThunderCats themes, incidental music from the show, and even commercials. They're all audio files, and some of them aren't that great, some of them have even been recorded off TV, like with a microphone, and uploaded- but when you're in a desert, you'll drink whatever water you can find.