Looks as if the replacement hood options are Sora 3500 and/or Tiagra 4600, I wonder if either of those are suitable. (I have two sets of the 4503 lever sets.) Both hood pairs are on ebay for less than $10 shipped.
edit: Looking more closely at the ebay pictures and my 4503 levers, it appears the 4600 Tiagra hoods are similar. I ordered a set, only $9.77 shipped. I'll report back when I receive them.
http://www.ebay.com/itm/Shimano-Tiag...kAAOSw32lYr530