Do you have a local machine and supply shop ? If so it's a great place for springs o rings screws and such. A tractor or farm supply may also have what you seek. You may need to take your dial caliper and size at shop to make sure you get the right one. Good luck.
I’ve had this problem with metric guns. The best thing to do is bore out the hammer to accept the .36” OD spring. There are a million options with this spring and it makes things so much easier.