Regarding a source of Digestives in the US: my friend and I found these at a local Giant Eagle Market District. I'm not certain how widespread their shops are (they're everywhere around here!), but their international section is really quite good.
Isnt it funny how mundane things in one country are hard to find exotics elsewhere? We really take our homeland food so much for granted.
I lived in Cyprus for several years (on UK military base) and it was considered absolutely essential that we should all have easy access to familiar UK food via the NAAFI supermarkets, but of course that costs extra due to the shipping costs so every service person got a really generous extra allowance to enable them to buy all that stuff. And this in a country with the most amazing fresh food available everywhere.