Not trying to be mean, but if you're not good at playing online you'll be hard pressed to make a lot of stubs in a timely manner.
You can always try to invest stubs in players you think will change tiers, but that's time consuming and very frustrating. Your stubs will essentially in the hands of SDS and their bizarre roster updates. Flip cards on the market, which is also no fun.
Completing career missions is a really good way, you get 14k stubs in the first tier and 9k in the second. So basically just play the game and do well and you'll accumulate a good amount of stubs over time. Try to work your way up to being a competitive online player and you'll have better luck.