You can get custom springs made for about 55 quid a pair in the UK. In the USA there must be something similar.
Start here

work out the spring rate you need on the site above.

Give the spring maker the length , Outer diameter, Inner diameter , and the spring rate you would like..

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod