Here's one option:
http://robart.com/collections/gear-f...a-a-cessna-310, but I see that the pricing
on Robart has gone through the roof!!!
Or Century Jet offers an nice set with several options:
http://centuryjet.com/shop/retracts/...en-cessna-310/
There was another company that made retracts for the Nosen 310, but I don't think they are still in business, you might contact
Lenny at Shindin Machine to see if he can make a set for you :
http://www.shindinmachine.com/
Bill D.