There are no geographic restrictions in Adobe's DRM. So once you buy an Adobe ebook from a USA store you are good to go. For ePubs, currently the best stores are Books on Board and
Diesel eBooks. I don't know which is easier to use from outside the US, but I'm sure both can be. At one time all you needed to do at BoB was register as a US resident buy gift certificates (with a UK credit card) and then buy the ebooks with the gift certificate. Another trick that works at some sites is to use a UK credit card and UK address but change the country on the credit card to USA (this works because the country typically isn't checked when authorizing a credit card transaction).
See also
Nvidiot's post about non-US access to Sony, although it sounds like you have already tried this.