I really want to do Motocross but I can't...

[jamjam]

I am 13 and love motocross. I've always wanted a bike but I don't know how to get one. I have no way to get money to buy a bike as they are all expensive. I also would only be able to fit on a 125. I have never really rode one before but I have ridden many little mini bikes that my friends have that are like 80cc. I have an old bmx I like to ride in the dirt and I always image having a dirtbike. I thought about sponsors and stuff but they wouldn't sponsor me obviously because I have never ridden a real mx bike before. If they would however, I would ride so much and do awesome things! Does anyone know where I could at least start because I have looked on Craigslist and stuff and I can't even get a $1000 bike. I have no idea where to start all I know is I want to ride real bad and would like any advice anyone has. Thank you,
Jamie