I have two. One I made myself: it was a bit of a hassle to get it right, but I still love and use it. I took an old gym bag and wrapped 2 sandbags in duct tape (sand from the garden store split into 2 garbage bags the length of my gym bag). Wrapped them each in the tape, then placed them lengthwise in the bag, wrapped in towels for cushioning until the bag was pretty stuffed. Make sure the weight is distributed well.
Next, I took 2 old leather belts from a second hand store and wrapped them around the outside of the gym bag twice. These were my handles. Made sure they wouldn’t slip off, but most gym bags won’t have sturdy enough handles to support the swinging motion. Learned this the hard way.
With the sand, my makeshift bag was about 25lbs or so.
The next one I bought was this model: it’s cheaper than the pink one and it comes WITH the filler bags (up to 40lbs). As long as you’re not abusing it, it should last you awhile. I’d recommend checking it out for wear and tear periodically and fixing it as you go! I’ve resewn the handles twice, but it still works really well (probably used it over a hundred times).
SKLZ Super Sand Bag
Hope this helps!