Older version of FCPX that will work with OS 10.13.6 ?

I have a 2010 MBP that I can't update beyond 10.13.6 and FCPX that works on a newer MBP. Any way I can get a copy that will work with my system?

You want to purchase the application? If so, there is no simple way to do this. You need to find someone with 10.14.6 to Catalina who is willing to let you use their computer to make the purchase. You go into the App Store on their machine. Log in with your Apple ID, and make the purchase. Once you've purchased the application like this, on your machine you can then download the last available compatible with that system.