It has been a few years since I have been cutting much at all but there are a few ready options that are free, or lower cost than the high price tatami.
Mixed in that pile of swarf are cardboard tubes. In that case very heavy wall but watch retail dumpsters for free material. Tall cartons in general. Something I used to cut a lot is just empty cracker and cereal boxes for those trick shots. Cardboard will scratch blades and tubes like above may have a significant amount of sandy stuff in it. The tubes above from rolls of pallet wrap and near a half in thick (which can be pretty brutal on light blades).
Pool noodles. They try to run away and require some technique at times but can be found in and out of season at a buck a whack. Beach mats. They can be doubled and tripled, then soaked just like other straw and reed mats. bamboo and softer canes grow almost wild in many parts of the country. Here in the NE, I have a border decorative cane that while softer than bamboo, can be bundled. Cheap plastic and rubber hose (without wire re-enforcement) can be cut to lenght and bundled. A very fun bundle or few were bundles spinning from a hanging line.
Rolled and soaked newspaper. Watch for staples in magazines. ABS pipe tubing. some parts of the country will have this softer black plastic, as opposed to harder pvc. FWIW, a friend used to sheath his mat cutting stand pegs with pvc and we invariably would cut some pvc at one point or another during the day.
There had been some stuff out there labeled "sword fodder" that was some form of packing material but I don't really know if that ever really caught on. They were the buzz a few years ago.
The most extreme and I still have it in a corner here somewhere is a pull on wellington leather boot stuffed with a 2-litre soda bottle filled with .50 caliber lead shot. A great stand up for poking and ultimate smacking but smears lead on the blade. I had though first put it together as a thrusting target.
Old rugs and clothing. again, soak as needed (or not)
On and on
Cheers
GC