Not much chance of buying a map in the US unless you can get a 2nd hand one.
Best bet is to pick one up in Vientiane.
Map is available at Wildside Intenational, Taipan Hotel, Kopchaideu Pub, PVO, the mini post office next to Lao Paris Hotel (next to PVO.)
Plus quite a few other joints around town.
Should not be a problem to score one.
If you're desperate wait until I get back to Chiang Mai & I will mail you one, - once I’ve got your money, but bank transfer fees will be more than the map costs!
Hope this is a help.
Davidfl
Keep the power on