The reason I've never noticed that you can buy RMs like this, is that I only ever used the RM market itself with the very obvious button in the CEO page. First I'd remove that button, after all, we have the RM market button in the main navbar too, when you try to buy a specific RM for a specific company, you don't need to see all the other options.
Secondly we could break down the buy method in the CEO page further, by using an input box below each RM, like
input box "buy resource" button "buy maximum" button.
So if you just wanna fill up the given resource in the company, you'd have to click the buy maximum button only, and it'd buy the cheapest offer from the market.
Clicking the RM icon itself would still bring up the possibility to choose where you buy it from.
Making the icons look like buttons so people notice it though it's a trickier thing, somehow we either need to make it look like a button, now it just looks like an image, so I have two things in mind, the first is the small question mark (like a tutorial help) in the say top right corner of each boxes.
For each individual box, these buttons would bring up a popup, briefly explaining the functions of the given box, in this case it'd show you the methods you can buy RM for the company.
Besides that, to make them look like a button, they need some kind of animation when you hover your mouse, so it'll immediately be obvious that it's clickable, either by making it look like a button, or/and use the tooltip for this purpose.
By the way, the tooltip now shows how much rm you have which is redundant, since you already have the value of the RM underneath the icon.