Hello Ian,
you bought yourself a wonderful device. I am sorry your first buying experience what not what it should have been, but actually it is pretty easy to buy content for your Sony.
There are two shops you
can't buy with unless you tinkerwith the DRM:
Amazon and B&N.
Every other story is fine as long as you buy epub. You can shop with Kobo, Smashwords, Sony Reader Store and a zillion others. Only make sure when given different file formats to go for the
epub.
Here is a sticky with other shop recommendations.
Hope you enjoy your new reader!