Lights can vary from one manufacture to another. For the most part, they will interchange but sometimes the plugs are different.
There should be a part number or manufacture brand on the lens. If not look at the back.
LED is the way to go anymore. They are expensive but great long lasting lights. If you have rectangular tail lights...LEDS are HIGH on those!
Incandescent ones are a lot cheaper but some lenses wont let you change the bulbs. I can point you in the right direction...Might even have some myself. Truck N Trailers is a GREAT source for these also.Talk to Matt and tell him Billy @ Tennessee Trailers told you to talk to him. Helps to take your rig there also.