You'll need a derailleur hanger - there are a plethora of styles but
this one might work.
There are many different derailleurs available also, and of course there are varying opinions on these. You need to be sure it is compatible with your freewheel gearing - (the range of gear teeth and length of the cage affect its reach) but you probably have a 5 or 6 speed freewheel, and there are many used Derauillers that would fit. My Raleigh Super Course has a
Shimano light action derailleur and it shifts very nicely. You can get one very cheaply on eBay if you hit it right.
In fact, if you find one like
THIS , it may be all you need.