the problem is I'm itching to expand it already!
Welcome to the addiction.
As was said above, there aren't many scenarios out there. Each of the large ships come with one and there's a bunch on the net. But no other official ones.
As a way to perhaps kill 2 birds with one stone... sorta Here's what I'd recommend as expansions.
The YT-1200 and Firespray models. Both come with more scenarios, you can get both for about $40 online, and they give you a chance to expand your collection. Best part is, they are pretty even point and power wise so you could add them in with out too much problem.
Next after that I'd recommend is another core set. With that you have more dice, a set of templates and damage decks for both sides, and 3 more ships for much less then you'd pay for them else where.
You can get both the core set, YT-1200 and Firespray from Miniature Market for $67 bucks or so, before shipping. Not sure what your budget is, but if you could spend that much money, that would be the best bang for your buck.
Edit: Forgot... There is the a Rebel Transport coming out sometime early next year, that I think will contain the Cinematic Rules which should have a bunch of new scenarios. That will cost $40-60 depending on where you buy it. But it might be the only way you can get your hands on those rules.