I use small pots from plants I purchased previous years. I've also used small plastic containers like yogurt cups with holes drilled in the bottoms.

I too save my plastic containers from plants previously purchased years ago. I just wash them well and store for use again the following year. Not sure where you are in Wisconsin but stores to try are the various Jung Garden Center stores in Verona, Sun Prairie or Stevens Point. I know the one in Verona sells sheets of plastic pots at a reasonable price (sheet of 1801 cell packs for example). Assuming they are open during this shutdown for covid 19. Or try Fleet Farm!