Medical offices, university labs, and doctors all unload optical microscopes for next to nothing (sometimes they go in the trash) as the world converts to digital imaging. Look for tag sales at colleges (these often happen in June), or simply advertise on Craigs list.
In terms of optical compound, I have a Steindorff from 1958. A Zeiss from '65 and a B&L from 1970's. (all less than $50 buys). Prefer the Zeiss.
A digital conversion is possible for one ocular of the binocular models of compound scopes.
The Dino-Lite digital medium power unit (10-90x) is really nice. Have one. Not enough mag for Nosema, but fine for mites. There are some lower cost clones of the Dino-Lite, quality is unknown to me, but reviews are very mixed.