just remove the bearing and bring it to your local autozone and see if they have any bearings with similar dimensions. there was a post about this before. i recommend abec 5 or 7. abec 7 isn't super super fast compared to abec 5 bearings, but it IS smoother and coasts longer than abec 5s. abec 5 is a little slower, coasts shorter, and is a bit rougher.
The x3 model has 2 bearings in each wheel, 2 in the middle sprocket and 2 on the swing arms. X1,x2 guys save a little when they replace all the bearings. Its good to repalce them the stock ones suck. They bearly spin.
The bearings are not that bad..but yeah there are not top the line..think they are like 680z bearings. Which come on cheap target, walmart skateboards. What i did to mine were soak them with tri-flow silicon lube also grease the sht out of the axle with auto bearing grease.