Congrats on your first marathon - its a slippery slope from here!
Yes and Yes! You can enter through the respective ballot system for most races -although remember not all sell out, so you can just enter!
Some of the bigger marathons - like Berlin and New York - have international partnership deals where they reserve a set number of entrants for international runners. But these are 'administered' by a couple of tour companies, like 2:09 Events and Sports Tour International - who effectively sell them with flights and hotels. Its expensive way to enter but at least you get a place - and your hand held when you're there!
I think London dopes something similar, but not 100% sure
Enjoy the next one!