The easiest and cheapest way is to use a late model R6 throttle tube. This requires no special cables, no special cable routing, no mods to the throttle housing and, last time I looked, cost only $7.83. The Yamaha part number is 5SL-26240-01-00.

BRG makes an insert to modify the stock throttle for $30. They list this for Suzuki's and I don't know if this works on an EX250 or not. I've also heard that a late model R6 throttle tube is a drop in replacement with a shorter turn for an EX250.

Mike Norman of G-Force carries Suncoast 1/5th turn throttle assembies that will work with an EX250 but, requires a small Honda start/kill switch as well. He has that in stock too and the wiring is all figured out thanks to David Crone.

Control pods
The control pods may be the same as that from a '89 ZX600