Toms35thSS

Recently had the recently rebuilt T56 (4,500 miles on it) fail. It destroyed pretty much everything to do w/5th & 6th gears. The shop that has it says it's not worth rebuilding because it broke 1 of the shafts, & is $800 new (says it's the shaft that 5th & 6th ride on? which is the main shaft correct?). They have been searching for a used 1 w/no luck. Also said the inside of the case is slightly gouged, but not enough that it would effect it. I know the shafts in the trans are the input, main, & counter shafts? I have looked them up, haven't seen 1 over $500, that's brand new on Tick Performance. I basically know squat about the internals of transmissions, is there possibly another shaft that isn't listed in rebuild kits? Does he just not know where to look? What would be my best option, seek another opinion or buy a new trans/search for a good used 1? The most upsetting part of this the trans only had 4,500 normally driven miles, on a 35k mile car. Also think it's possible it failed because of who rebuilt it 4,500 miles ago. It lost 5th & 6th on cruise control btw. Thanks for any help or advice, would be greatly appreciated!