Here are several places where you can buy from stores & online websites:

Several places to buy Julmust:
- IKEA stores (Rarely available-  IKEA store in Boston, Mass.)
- Wegmans stores? Yes they do have Swedish foods & stuffs but I have never seen any (not even one) Julmust drink.
- http://kristallbeverage.com/home/jul.html (Official Website- the online store is always down *offline*)
- http://www.scandiafood.com/food.aspx?highlight=7
- http://www.simplyscandinavianfoods.com/foods.html
- http://www.worldmarket.com/product/index.jsp?productId=3807621
- http://www.scandinavianfoods.net (not sure if they do)
- http://swedensbest.com/julmust.html

Just do a search for ‘Julmust’ at Google.com.