How can I purchase war assets for in-game money? According to this question it should be possible. Or if not, where can i spend it? I found only little ways to do it. (The Citadel is under Reaper control, so I can't go there)
There are a couple side missions that require you to buy an item from a store and that end up rewarding you with War Assets, but I don't think you can buy War Assets directly. After all, you can just barely afford a few guns. How do you expect to be able to pay for whole fleets?
As far as I can tell, the only ways to earn War Assets is by completing missions and side quests, or by scanning planets.