How to fill it? Those big metal clothes donation dumpsters, people drop off unwanted clothes. Those things are often overflowing, people just leave bags and bags of free clothes. If it sits outside too long it will just get thrown away anyway. If you do go this route, bring your own large plastic bags and a pair of rubber gloves. Take clothes out of the bags/boxes you find them in. Briefly inspect to make sure there aren’t fuckloads or bugs inside. put all clothes in YOUR bags OUTSIDE OF YOUR VEHICLE, tie it tight. Bring all these clothes to a laundermat, wash everything with bleach, dry on high heat (this is imperative). When they are done, put them in a NEW, UNUSED BAG before taking them home. Doing this exactly as described will ensure you don’t get things like fleas, bed bugs, roaches.
There’s a ton of videos on YouTube that show you how to fill your own Muay Thai bag. Best thing is to cut fabric into 4 inch strips , but the actual process of filling it, check out the videos.
Otherwise, buy a filled bag, which is going to be a whole lot more expensive. You can buy filled Muay Thai bags for a little over $100, I would not recommend that. You get what you pay for. If you are lazy or just have more funds, fairtex filled bag, not prime, but free shipping..
I’ll say it again, you get what you pay for.