Try this site:  https://www.theworldmodels.com/store/?action=product&sub_id=216&subsub_id=269&filter_index=valve+springs

Also, you can find quite e few parts on eBay.

Had thought it might take a bit of reverse engineering and that I might perhaps have to look at making some springs if I cant find anything that will work otherwise.
The remaining FS-20 spring I have measures up at 11mm long with a 6.2mm OD, and 4.8mm ID. It is wound from 0.7mm wire at a 2mm pitch and has roughly a 290g/mm spring rate but I do need to measure that more accurately.
The FS-30 springs are shorter but wound out of 0.8mm wire at a slightly coarser pitch. As far as I can tell the compressed lenght they operate at when they are installed is the same in both motors. I should have measured their spring rate when I had them out of the engine but hopefully after a lot of searching I have found a couple of OS FS-30 valve springs and they are on their way to me from Singapore. As long as the spring rates are not wildly different and the force they provide at their 8mm compressed lenght is similar by all accounts they look like they should be fine to use as replacements in the FS-20 as well.
Since my first post I have also found Just Engines in the UK have some FS-30 compatable valve springs listed on their website so will see if I can get some from them and see how they measure up as well.
Will keep an eye on eBay and see what comes up there in the future if things don't work out with using the FS-30 springs in a FS-20 but I am hoping given there are not alot of differences between the valve trains of both engines and the operating RPMs are similar they will be OK.

 

There was a post or two on another forum about the FS 30 valve springs breaking but I am new to running fourstrokes so missed most of what was happening with the FS 20s and 30s back in the 80s and 90s when they were most popular. I think breaking valve springs most likely isn't a problem for the majority of flyers who are running these engines and looking after them well.
For a bit of context the FS 20 I have looks like it was well used until the model it was in secumbed to the overwelming effects of gravity and the engine was then thrown in a cupboard many decades ago. It did take a bit of work and a few new parts to get it back running again but bringing these engines back to life is half the fun. 
The down side of it not being oiled up properly before being stored for so long is that many of the expose surfaces of the steel components inside the engine were covered in a thin layer of surface rust. It was easy enough to clean this off with a chemical rust remover but I'm taking a guess that what might have caused the problem with the valve spring in my engine is that the corrosion had been more severe in patchs on the surface of the wire and created some weak spots and that is where it failed. There might be another reason but thats my thinking for now.
Hopefully a with couple of new valve springs the engine I have will run like new for a good number of years again.