I don't really play rs3 anymore but ive always thought farming herbs was a good source. Other then that pvm is now the best source of money I think so once you are able to afford gear that should be the way to go! Gl mate
I would try looking into guides put up around 4 months ago. I find that if you look for the most recent guides, those methods will be hoarded and the profit will decrease by the day. If you find something that's back at a stable point and you're one of the only ones doing it you can make a fair bit of cash.
It may seem like we don't want to tell you, but if you find some less well know ways yourself then its all yours and it will make you more money.
But Flipping is great if you get enough cash its a way to make a steady flow of money by sitting around in the GE
Goodluck mate
Real knowledge is to know the extent of one's ignorance.