3 prong with one flat taller than the other? Lowes.
I used a flush mount wall plug on the end of my extension cord, this:
http://www.lowes.com/pd_71246-334-1..._prd_lis_ord_nbr|0||p_product_quantity_sold|1
The back is enclosed and it has a cable clamp, I took an RV power cord and put the 220v ends on it to make my extension cord.
Another option is to use a regular 50amp outlet in a metal box clamped on the end of a cable.