Rosco gel (color filters) comes in 20 x 24 in. sheets. Costs about $6 from B&H.
http://www.bhphotovideo.com/c/produc...er_Medium.html

BMI Theatrical supply sells it in rolls for about $97 per.
http://shop.bmisupply.com/ProductDet...27--Medium-Red

Use Rosco #27 (Med. Red.)

For safelight, I use a double layer of gel, just to be sure.