I shoot Straw Bales, but I covered them up with some thick Plastic sheeting to protect the Top from Rotting. Been 6 years and no complaints yet. If you have a  Semi Truck Repair Shop close, ask them for their Old Mud Flaps. They just throw them away anyway, and put Their Name or Logo flaps on when they send them out all repaired. Work like a Charm!!

A friend of mine uses large sized conveyor belting. Arrows mostly bounce off or just barely stick. And I've been trying to get my hands on some mudflaps from work. They're for mining trucks and they're about 7 feet by 6 feet by 1/2 inch thick.
Dave.

I just went back to hay this year. I haven't done anything to it (like I should have) and it's holding up fine. Cost is about $12. a year, including the plastic to cover 8 bales. This fall, I will put the bales into the mulch pile and replace.
We are shooting maybe 100 rounds a day and with bales rotation every two weeks along with moving the target face around, we dont get any shoot through.
Bows we use are 45-75#

I suppose (eventually) we will have to stop composting and do it right but for now it is working fine.

If you decide to go with straw squares you can use metal blunts?

Years ago I could get chip bales, they are made of wood chips and came in squares. Don't know if they are made anymore.

Lumber mills is where I would look. Oh they can get messy when they start to fall apart.

I use the big round bales, when i wear one out I just call my buddy and he takes it home and brings me another one, guess I'm kinda fortunate! But if I didn't have those bales I would use the rubber stall mats for a backstop.

I use straw bales for 90 % of my shooting for the last 30 years...  Field points, broadheads..  you name it....  2 broadheads can normally be pulled out easily, depending on model, 3 blade heads, I usually pull through....  They can be a bit hard on fletching, but thats a trade off I take up.;;  I band them with a regular stell banding tool and will tighten them up on a yearly basis...  3 bales last me two years, depending it the weather stays off of them in the off season...