The only 'real shop' you're likely to find Velvia 50 in 4x5 is Japanexposures, and they're not exactly cheap for the rest of their stuff, so combining postage is a bit pointless (and they don't have much Kodak/Ilford stuff). I got my *new* (Jan 2015) RVP50 from this guy on fleabay, for "only" $7 a frame. But all the rest you can easily get at B+H / Freestyle etc and combine.