My preferred solution would depend on the style of freehub. If it's the later style, with the body that is threaded into the hub shell, then I'd search out one of the 7 speed, Hyperglide bodies and install it on the existing hub shell. That way, you still have 126mm spacing, at a reasonable price, have one extra cog and can use readily available Hyperglide cassettes without worrying about the availability of the threaded Uniglide outer cog. Of course, the unknown is whether the OP is running indexed shifting, which would involve other changes.