REI, because of their mail-order business and multiple retail shops is a good bet to have exactly what you need IF you contact them IN ADVANCE and make arrangements for it to be at the specific REI store you will go to.
Another possibility in the Boston area is Harris Cyclery in West Newton, Mass (a Boston suburb). Harris was the shop the esteemed Sheldon Brown worked in and they are a very well equipped and well stocked store with the ability to get almost anything you want. But, be sure to contact them in advance too.
Getting a couple of longish rides in before starting out is a good idea as the selling shop could do any adjusting you need before you are too far away.
After than, any shop can adjust any bike but they won't do it at no cost if you didn't buy it from them. If you haven't developed at least a medium level of bike mechanic's skill, now would be a good time to start learning. One thing you will discover about the US is that there are a lot of really open spaces so being fairly self-sufficient is a great asset.