The Suzuki parts list has the standard gearing at 15/40T. Like you said, way, way, way off what you need.
I'm running 13/42 ( 520 chain )( you get to use all the gears ).
you can get sprockets from any bike shop that sells JT Sprockets.
13T front sprocket is JFT 431-13
fits:...RM250 A/B/C/C2 - RM370 A/B - RM400 C - PE250 B/C/N/T/X
other available sizes 11,12,13,14,15
42T back sprocket is JTR 820
fits:...DR250 1982-87 - SP370 - SP400
I can't remember the exact prices but they were cheap.