If you’re in the UK, John Lewis sells some great own-brand loose sheets and envelopes. I used them a few years ago for a calligraphy commission. They were slightly textured but not enough to disrupt the strokes of a dip pen so they should be fine with a foundation pen. No bleeding, see through enough for a bold guide sheet. I’ll see if I can find a link.
I’m not sure it’s special enough for that. But if you can’t find anything better I could buy it for you and send it personally if you like? You wouldn’t need to order too much that way. Send me a DM if you’d like me to help 🙂