Synchro ring and slider is the same for all World Class T5 transmissions. The 3rd gear is available OEM from Tremec, but the 4th gear (input shaft) is not available anymore, so this you would have to sort. 3rd gear is 30 tooth. (It's a part of the 2,95:1 gear set often referred to as Heavy Duty T5.) I used to have a stock of new input shafts but I'm all out. Maybe Bara could help you out.
A lot of cars were fitted with T5s but there are many different ratios.