There's simply no sources for them outside of the classifieds and even there they would be rare. The only JGs you can find on online stores accessible to everyone are the Dragon Maul type ones since they look like toys (but underneath they are M4s).
And yes, both are all 'technically' TM compatible. You learn quickly in airsoft that compatible means differences in tolerances and fit and often entails modification or trial and error or busted parts and busted guns and more and more money thrown in. Therefore, I would agree with The Saint's advice and just game your stock gun if you are new to the sport. I know when I first started I played about 10 games with my JG before the arms race started and I felt that I needed to upgrade and I must have blown $200 in parts that ended up not working or busting or giving me nightmares everyway you can imagine before I got it together. These were the older version of JG guns with the nylon bushings and worse tolerances and everything. Upgrading was nightmare.