The screen doesn't keep mosquitoes out. It keeps trash and debris out.
I wouldn't buy one built from craigslist, or whoe foods. They start at around $50 and go up to about $150. It's very easy to build one. You buy a barrel. You can buy one from $5 - $15. If you look around you'll find one. A faucet can be bought from Lowe's for $5 and a 0.50 pvc nipple. You cut the top off of your barrel, drill a hole at the bottom for the faucet, put some silicone around the whole, screw the nipple into the faucet through the hole. Let the silicone dry for 24 hours and your all done. Just set it under a gutter, we installed a big piece of found curved metal along the side of our coop. You'll want to cover the top with window screening to keep out leaves. It's not a hard project at all and anyone can do it.
ED: If you can't find rain barrels in your classifieds, or for sale by the road, like you can around here. You can buy them from soft drink plants. I'm sure there are other plants that sell them used, you just have to call around.