You wont find information what to buy and for what price anywhere cause that would go against the whole reason to do it. There are different types of merching, quickies or long term. Anyways you have to find your own items based on past experience or foresight. Let's say a Jagex posts come out about how they're gonna make you use X item for Y thing. You now know that item have a higher demand so you buy yourself a bigger supply for the future market alongside with the higher profit margins.
The more popular variant is flipping when you just find an item you can make some quick profit off, either be it knowing about differently used items with decent prices or just brute forcing your way about different kind of items. It's very simple and straight forward. You search for an item and look up the highest price and lowest of it through the Grand Exchange and then put an offer in for the lowest, when you get those items you put it back in for the highest amount, profit.