For starters, you could get the lens out of an optical drive and place it on top of a smartphone's camera and you'll get ~50x magnification, although with a tiny depth of focus.
There are hobbyists claiming up to 300x (at this level you can clearly see bacteria), by stacking two lenses.
In any case, there are extremely cheap DIY solutions out there, but it may not be worth it for you.
Some ideas: https://www.wired.com/2011/03/diy-cellphone-microscope/