I got it on Amazon over 6 years ago (was only $20 than 😔 ) but if you look up web search canvas backpack it should pop up. Bet you could find used on depop used Gootium 21101 Specially High Density Thick Canvas Backpack Rucksack https://www.amazon.com/dp/B00AV4Y0IS/ref=cm_sw_r_apan_glt_i_S3BWWWG4CXYF5AEEKKHP
I went to a military surplus store and found a canvas backpack similar to this guy:
It's kind of adventure-y! Hold entirely all of my shit: Minis, books, rolled up maps, dice, pencils, everything. It's pretty heavy duty, so I take it on camping/hiking trips as well. The one on amazon is 40 or so bucks, but at the surplus store I got it for like ten, so look around! Good luck!
This sort of stuff is common.
Gootium 21101 Specially High Density Thick Canvas Backpack Rucksack https://www.amazon.com/dp/B00AV4Y0IS/ref=cm_sw_r_cp_api_glt_fabc_RBFVVFW6S8CT5WP8SQES?_encoding=UTF8&psc=1
It is an outstanding backpack. Definitely worth purchasing if you need a durable cloth bag.