The moss you can occasionally buy is rarely the type of smooth moss you want for your Japanese garden - it's usually sphagnum type which is taller and straggly - lovely, but not the smooth green that I imagine you want. I'm surprised there is none in your garden already, there is in mine! Try woodlands and hedges where there are stones and logs, it may be growing there. It is not illegal to take things like seeds from the verges, but you may not dig anything up there. I cannot imagine moss would be considered a problem. Failing all that, there are Japanese garden centres around, maybe one of those could help you? Have you tried on line, maybe search engine Japanese garden supplies?