Plastic soldiers are exceedingly easily sourced from just about any store, even most grocery stores have some lying around. However, plastic soldiers have a very long history going back to 1938, first being made by Bergen Toy & Novelty Co. Since then, many companies have risen and fallen in the manufacturing process (Louis Marx and Company, MPC, Airfix, Matchbox, etc). You can still quite easily find these older brands at shops such as the thrift store, yard sales, and flea markets. A great source for minis nowadays is BMC Toys which carries a wide variety of lines and also has the molds for many older lines, keeping the history of plastic soldiers alive. Check them out!
The common scale plastic soldiers come in is roughly 54mm (1:32). This can vary pretty wildly depending on the manufacturer, so be sure to check that the height of the figures is what you expect when purchasing. Of course, the game can be played with standard 28mm figures with no rule changes if this would be more convenient.
Plastic soldiers come in a handful of "standard poses" such as "grenadier" and "radio operator." Which poses you pick to play with does not matter, as long as both players get to use the same poses. Currently, no special rules are implemented for the different standard poses but is in the works.
The majority of plastic soldier come in brown and green, which is sufficient for differentiating them on the game table. However, there are some simple steps that can be taken to improve their appearance. The simplest approach is to use colored sharpies and drawing directly on the soldiers, which tends to take the ink well (do a test first before committing to the approach). If you are playing with kids, this might be an easier approach than paint (though sharpies carry their own risks!). Next up in complexity is to use a wash to add shadow to details on the figure. Plastic soldiers tend to be made from LDPE plastic, which doesn't always take paint very well, so it is useful to begin with a spray of clear matte varnish to give a better paint surface. Of course, infinitely complex painting techniques can be used to enhance the visual appeal of a plastic soldier, but that would destroy the lofi appeal!