There isn’t a Rookie League per se. However there is a forum league that is broken up into 5 different tiers. The tiers from top to bottom are M-E-1-2-3.
Here is the “Find League Opponents Thread” that you can use to find league opponents:
Here is a link to the league rankings sheet:
I would suggest to start by reading the league rules and then posting in the Find League Opponents Thread and asking for either newer players or those in rank 2-3 to see if someone is up for a game. You can also look back through the list of people who have recently asked for games and try hitting them up with a personal message to see if they are still looking.
Alternatively you could also try looking at the league ranking sheet for 2020 and send a personal message to some of the lower tier players that play often (have completed many games) and see if they would like to play you.
One thing to keep in mind is that the majority of the league games aren’t actually played using the traditional out of box G40 rules. Most games are played using either the Balance Mod (BM, or BM3) or Path To Victory (P2V) house rules. So, keep in mind that if you’re only looking to play the traditional rules, that might disinterest some potential opponents.
Hope that helps.