While I can cover the Washington DC ones (and do here), I don’t know too many other tri shops outside of the DC area. Your best bet is definitely Slowtwitch for this one, the forums are a wealth of information.
One thing to keep in mind, is that if you’re looking for technology gear – you can have it direct shipped to your hotel in the US. Meaning, you can order it on Amazon or wherever and simply ship it straight to your hotel. I often do this on business trips where I’ve ordered something I need mid-trip. Simply fill out the address as such:
John Doe (change to your name)
C/O Sheraton Hotel (change to your hotel, but keep the C/O in front – which means Care Of)
123 Main Street
City, State, Zip
That’s it! Super easy, it’ll just be waiting for you at the front desk when you arrive.