There are 2 types Home made and store bought. Right now store bought is selling for 99 bucks and that's a great price. Don't get the bat wing you don't need it.

This is the cheepest place I have found todate for store bought ones. http://www.mdhydro.com/ht-reflectors.html

If you are only using HPS then you can make them with Bake-A-Round Pyrex glass from eBay. The final cost to make one is like $30-40 bucks and it's only 4" so it will only work for HPS. You get better light coverage and more lumens with the light hung Vertical too. ( I bet you already knew that) A reflector not only deflects light it also reflects HEAT.