Well, I don't know where to get them reliably.. but I can say that
www.coin.demon.co.uk has an Atari parts section and a RoadBlasters CP (or they did awhile back) for sale.. personally, though, I didn't order it because the shipping would've brought the item to over $100 [to the USA]..
It seems that the majority of the yoke parts are unique to this controller (and its 'clones') and nobody really produces refurb/repair kits anymore.. this is why when they do actually appear on ebay they are expensive -- last kit's 'buy it now' was set at $60.. for a few switches, bolts, plastic gears, triggers, and rubber switches, this seems a bit high to me.
To replace my own triggers (both thumbs and one finger), I bought and harvested a RoadBlasters CP.. the handles [only] are 100% the same (except for the size of the thumb triggers and their mounting holes -- more sturdy, though).
k-spiff