Grabbed a pack of Casemate index cards at Wal-mart today since I can't resist back to school supplies. These are about .58 cents a pack right now:
Now they're clearly not ideal, but they worked at least passably withall the inked pens I had hanging around with no bleed-through to the back. The Diamine even still showed some shading. I'm betting they'll behave like a charm with X-feather, since they took 54th Mass from my M nib TWSBI 580 with no issue. Copper orange behaved well too, with less feather even than it usually shows on my Rhodia pads (I don't know why, but it does.) My scanner is not the greatest at capturing true to color. Sorry!
For other folks who go through index cards like I do, might be worth it to stock up on these. Sorry about the crumpled up label, I was just going to use these with gel pens like I usually do but I thought 'what the heck' and tried some FP's on there. Happy bargain hunting!