You can buy them in Halfords, Atlantic etc, Turtle Wax do a good one. Be very careful though, if you leave them on too long they can damage the finish on the alloy.
A good tip is when the alloy is clean and dry give it two coats of car wax and polish off. This creates a barrier between the alloy and the brakedust\dirt and makes it easier to clean next time.
Isn't wiping the alloys with cloth dipped in diesel or petrol do the same job? - then rinse well with water and car wash??
I remember my cousin got a 5 gallon drum of stuff from his mate in the motor trade. This removed tar etc very quickly - cant remember what it was though