Just buy a flip flop hub, put a freewheel on one side and a fixed cog on the other. Then find an ebb frame and run it with a quick release hub so you can change it immediately you need to.
If you don't fancy the convenience and simplicity of that then there're frames with either sliding drop outs or horizontal dropouts or track ends but they're, in my opinion, P.I.T.A.es, especially for trying to do any adjustments quickly.
Phil wood make many different hub options including fixed/freewheel -
http://www.philwood.com/Spechighflangehubs.htm . There are other options though.