Hi Matt,
Personally I have no experience with the RT125, but I believe Dave Thompson has had one, for many years, and loves it! From the specs it seems quite a decent 125 4T.
While the traders mentioned in
viewtopic.php?f=5&t=9257&p=47896#p47896 are mainly focussed at the two stroke bikes, you might find some spares there; look at their websites.
Beyond that: search the web.
Good luck & enjoy the bike!