You sure can not make real separations with ortho film, no matter how many
filtes you sandwich in there. I don't think there is any inexpensive
film available in large format size, but Samy's in Los Angeles
(1-800-321-4SAM) has Ilford FP-4+ and HP-5+ available in 7X17, 8X10,
11X14, and 12X20. The 11X14 is priced at $91.50 for a box of 25 sheets,
with which you could make 7 or 8 separations sets. With three-color gum
this should last a long time.
Sandy King