Third rail is very much a specialty item, so it's pretty unlikely that you will be able to find it ready-made. Model Memories makes hangers for third rail, it appears that it is intended to simulate "under-running" third rail rather than the "over-running" style used in subways. Your best bet is probably to get some bulk rail stock (probably in a code lighter than you're using for your track) and run it alongside your mainline, using a bit of tie every tenth tie or so to secure the third-rail. Not much work, pretty cheap, and it would look pretty close to prototype, so there really isn't much call for a commercial product.