If you have a sunny window available to you, it's pretty easy to set up an algae farm. I did this for my otocinclus with a 2.5g tank. Filled it with tank water, tossed in about an inch of polished gravel seeded with a bit of substrate from one of my active tanks, added a half dozen relatively flat river rocks (as many as would fit without shading each other) and put it in direct sunlight.
Took a couple weeks to get much algae growth, but once the rocks started turning green on top, I'd pull one out every couple of days to add to the main tank. When possible, I'd pull the rock out before they'd grazed it entirely clean as the algae regrew much faster if there was still a bit left on the surface when it went back.