indoor i use the stock AKG screens
outdoors I use the large DPA UA0896 screens for most uses
really gusty winds i use the Big Ass Shures A81WS
not sure of the cheapest place to buy them, but they can be found at just about any online retailer (B&H may be a good place to start) or do a froogle search. they can be found here used, from time to time as well.