Pearl Art Supply or any retailer that sells graphic arts materials usually stocks ruby or amber lith and rubylith tape. You can also use a UV opaque colored art paper, black or red for example about 40lb weight.
Originally Posted by photomc
Most times I will mask the negative to give even white borders on the print, sometimes however I will add a quater inch or so on each side of the mask opening to create a DMAX border which will be seen when the print is matted.
I also try to cut the outside dimensions of the masking material to match the paper size unless it is a pretty large print. This way your image will be centered on the paper.