Amayama and megazip have them (cheap).
Recently bought syncro 2 (336720060) 3 & 4 (same part number 3336820040) from amayama. Amayama UAE had them cheaper ($30 vs $55 out of Japan) higher postage from UAE but I got bulk parts, took about four weeks in total to receive. Before you buy ensure you have your gearbox open for if you have single or double syncro, as well as what other parts you may need.
I am about to order other parts, because once you open the gearbox there are more parts that need to be replaced than just syncro: seals, snap rings, keys, screw barrel retainers, bearings, and other bits you come across like input boot dust cover etc.
Parts are available however have superseded part numbers - got them from the help of KO Racing USA.