Duncan (thewatchbloke) is the best guy to ask, he's got lots of Seiko experience.
I've never been able to find a suplier for Seiko parts, I don't work on them often. Regarding the mainspring, it may be possible to measure it and source a generic from Cousins. They list a lot of springs, if you know the height you can see what' s available. One rule to remember: the strength of a spring is proportional to the cube of the thickness, so a 0.105mm spring is 15% stronger than a 0.100mm.........worth knowing if you can't get exactly the right dimensions. The height has to be the same to fit the barrel but a couple of centimetres in the length won't have much effect.
At the risk of stating the obvious a good quality digital caliper is a must, if you don't have one I suggest you get one.
Ideally, you should source the correct spring from the manufacturer but sometimes it isn't possible.
Paul