1. Buy the foot warmers made to stick to your sock. They are thin and do not require the same amount of air flow to keep them warm.Attach one to each layer of sock (i usually use only one per foot bit since your boots are uninsulated try 2) $1.50

2.make sure your socks have no cotton in them. Wool works great.if you have a polypro liner wear that against your skin. If not you can pick one up at wallyworld for $3

3. Spray anti-persperant on your feet. Dryer is warmer.

Of course this is all based on the fact you have a very limited budget. Otherwise you could also add woll/felt inserts into the boot but it can cost a bit