Most Helpful Guy
Do you mean real motocross jerseys as a fashion statement? Or do you want to take up motocross? (No judgement; just curious.)
Fox Racing is probably the most ubiquitous. Here is an excellent list of riding gear brands:
You can get a lot on Amazon and other online outlets.
There is also a brand called BiLT, which is sold only at CycleGear franchises and online. It's very common and reasonably priced. If you're doing this for fashion, maybe you want to avoid it, but if it's for riding, go ahead.
Most Helpful Girl
Well if you are actually serious, something like this: