I made my own. I cut a piece of 3" x 1/4" steel, which has angle bracing on the edges to make it stronger, and I cut it 3" long. I drilled small holes in the steel to screw it fast to the bottom of each leg. Then I drilled a hole big enough for a 1/2" bolt to fit through. I drilled the same size hole in the leg for the bolt to screw up in to. I hammered a nut into the leg. I put a nut on the bolt, all the way to the top, and added a really thick washer on top of the nut. The bolt goes through the hole in the steel and screws into the nut that is embedded in the bottom of the leg.
I bought the steel at home depot, along with all the bolts, nuts, and washers. Using these, the stand is 100% completely level in every direction. And with the large surface area of the steel on the bottom of the leg, and the large washer between the steel and the nut, it's not going anywhere. The stand didn't budge or groan or anything when we put the 800 lb tank on top of it. Granted, it's going to get another 3,100 lbs on them but I really doubt if it will be a problem, especially when you consider that the 3,100 lbs (3,900 total) will be spread out across 8 legs so it's only 490 lbs per leg.