You can get pure oxygen cannisters at Walmart, not sure if and how you can use them though.
As far as top heavy, it would depend on area of the base vs. how high you filled it, I think-- the height of the center of gravity vs. mass. As far as better than a tub for sloshing, not certain how much the circular geometry inherently dampens sloshing vs. the simple dimensions of height vs. base area. I think it might, not certain to what degree. I do know that filled buckets, paint, for example, are tippy. Not a great idea to leave them loose to tip and roll around because on a tight enough curve they will. Half-filled bucket for fish are less so, but I still like to pack them in some how so they can't tip.I’ve heard that as well! It does make sense because there is less horizontal distance for the water to gain momentum. If only they made 10gal buckets at Home Depot that were slightly larger ... there are trash cans however! Do you think a cylindrical trash can would work better than a rectangle tub or would it be top heavy and not stable in the car?