3rd gear problems! HELP!!

Anyways, now, car wont go into third at any rate of speed, it grinds and makes all kinds of terrible noise. At a stop i can put it in third and bog it and when getting up to speed it will stay in gear, so i know there is no damage to the gear itsself. Im thinking shift fork or synchro problem? Someone let me know. Im thinking its the fork because i know that the 3-4 fork is aluminum. If so, does anyone know where to get a replacement stronger fork. Im new to the mousetrap world. Thanks fellas.


