Kids' drawing tablets are often made from a cheap paper like newsprint. They are usually about 8.5 x 11" so a good size for string quilt blocks 8.5" or smaller.

I can these tablets from a local teachers art supply house. They also have a 500 sheet count pkg, I think that I paid $10-$11 for.

I use thin, inexpensive muslin for string blocks that way I don’t have to remove anything.

Found a backing at our local thrift shop unopened with no size on the front but for $1 what the heck. Just opened it the other day and found it thin muslin cut into 2 - 36 x 72 with an extra piece. Can't use it for a backing so decided I'll use it the next time I need a foundation on the back. Its not good for anything else.

I always use newsprint paper as a foundation ... also use in paper piecing. I have purchased newsprint paper at stores that sell teacher's supplies. Years ago our local newspaper would give you the end of the roll they used when they printed the newspaper. It was quite wide and free! I'm not sure they do that anymore.

I buy my deli sheets at GFS and they're less expensive. Also available at Costco in a 2-pack even cheaper yet and then you have one to share with a buddy.
Don't get me wrong, I'm an Amazon junkie too (and I think they need to start recovery programs for us :o )

I like to make my string blocks 10" square and I use full size paper towels (I think they measure 11.4" x 11.4") I trim them down after I sew all my strips. Not too expensive and they work great for me.

I looked, and you can buy whole cases of different widths for $35-$50.

Our dollar store, specifically Real Deals usually carry childrens drawing pads for $1 each with that newsprint type paper.

Also, I frequently repurpose bed skirts from the thrift store and the thin mattress cover part is great for foundations. Sometimes I even use them for t-shirt quilts by quilting the tshirt design to the thin fabric, rather than buying the adhesive backings. You get a lot of foundation squares from a king size bed skirt top!

I've always used phone books. The older the better. New ones can smudge a bit so I use books that are two to three years old.