Here is a link to one possible tail light fixture replacement:
https://www.etrailer.com/s.aspx?qry=...Trailer_Lights
Before purchasing replacement fixtures, check the ground for each of your current taillights. Use a jumper wire to go from the socket base to a known good ground. If the lights work properly the connections/sockets may just need to be properly grounded. You may not need to replace the fixtures.
Dick