Those look like standard 5L mini kegs to me. They can be bought from many beer supply stores: midwestsupplies
. You can also recycle commercial kegs (available at many/most liquor stores.) Around Ohio you can buy a 5L keg of commercial beer for around $15 - less than the equivalent in bottles - so it's quite a deal if you can find a beer you enjoy in one.
Since they are used for distributing commercial brews they'll hold pressure just fine; Although I've heard that you do need to be careful if you're using them for forced carbonation.
The 'drain' is actually a tap that can be used for serving; you need to relieve the pressure in the keg or it will spray foam everywhere. The commercial kegs have an air valve integrated into the top bung to vent excess CO2 when you start serving, as well as to let air in as the beer is served to equalize pressure.
Anyway, 4 of those kegs adds up to about 5 gal of beer, so I'd guess that it's intended to be used to keg and naturally carbonate a full 5gal batch of beer, with the CO2 system used for serving. Not a bad little setup. How much did you pay for the tap?
The main knocks on them for homebrewing is that the inner lining wears off after several re-uses, they can be a bit fragile, and you need to buy or make non-standard fittings to serve from them.