Is there any repo that will let me install mysql 5.0.51a on Ubuntu 10.04, or do I have to install it from scratch?
Add this source to your sources.list:
deb http://security.ubuntu.com/ubuntu hardy-security main
One thing you can try is to install the .deb file from an older version's repository. It appears Hardy's repository has the .deb you are looking for. One issue with this method are dependencies that might pop up for libraries too old or, in this case probably, too new for the version of MySQL you want to use. Wouldn't hurt to try, though.