Amazon has "military grade duffels" that hold up well so far. I bought this brand and have it currently loaded at ~180 and it has held up well getting dropped from waist height for several months.
Bought it off of Amazon here at this link. I got the 25x42 which works well to simulate the size of the atlas stones I need to use, though I've wished a few times I also got the size down for longer distance carries. Its currently loaded to ~220 pounds and it has so far amazingly held up to a lot of carries and load over bars onto a crash pad. Been using it for about a year now. They are really versatile too. Just last night I realized I could use it as a pad for GHD too.
I use one of these from amazon:
Rothco Heavyweight Top Load Canvas https://www.amazon.com/dp/B00029R84K
Just filled it with sand. Been between 180 and 220 lb since June of last year using it for carries and load over bar