Anaheim Gear here in So Cal was a source for the syncros a couple of years ago but they no longer have them. I get bearings and seals from them and an occasional shaft but that's about it. I rebuild a lot of OD transmissions and my best source is finding regular gear boxes at swap meets and Craigs List to use for parts. There are other wear points for the popping out of third gear problem as well. Worn 2-3 shift fork and the engagement spring on the clutch gear are also problematic. Jerry's offer is probably a good way to go.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine