I use something similar to this to hook my pi to an old wood grain tv with a single coaxial input, but there are hundreds of different brands.

https://www.amazon.com/RCA-Compact-RF-Modulator-CRF907R/dp/B0014KKV7W/ref=sr_1_1?s=audio-video-accessories&ie=UTF8&qid=1486677432&sr=1-1