Another idea to add, I recently ordered a spring from Cannon and it got to me within a couple of days. $119.
Get out your caliper/tape and measure your spring I.D. & length, and they will likely have what you want in stock.
(Try a YZ250 match, 59mm x 270mm. This was 10mm longer than my stock spring, but it fit just fine. Also, if you haven't measured a spring before recognize that the top and bottom I.D.'s are slightly different, in this case 59mm is the smaller end).
Be aware that they measure their spring rates by Kg/mm, and GasGas (at least in the 2018 manual) uses N/mm - which is slightly different. You want to make sure you're getting the new spring rate you want!
Jeff
https://www.cannonracecraft.com/contact
405-524-7223