Although this is far less common than running into a wobbly table, it can be just as annoying. And that's wobbly toilets. Luckily, we have a pretty easy hack to fix that. All you need are a couple of coins or washers. Just shimmy them underneath the toilet, adding or subtracting as needed until the toilet seems stable. Then sit on top and attempt to rock the toilet just to make sure it's stable.
Finally, using a caulk gun proceed to caulk the gap and hide any part of the coins or washers that might be visible. This will save you some money in having to call an expensive repairman.