You can get Phoenix Worms directly from the main site:
http://www.phoenixworm.com/servlet/StoreFront. The shipping fee is constant, no matter where the order is shipped. That's your safest bet, as these are directly from the company.
If shipping costs from Mulberry Farms is too much, consider ordering hornworms (and/or superworms) from here:
http://www.greatlakeshornworm.com/. Based on their name and phone number's area code, I would assume they're located in Michigan, so shipping costs and times should be great if you're in the area.
Otherwise, most local pet stores should carry superworms. Phoenix Worms and hornworms are difficult to find at local stores, particularly at large, corporate pet stores. You may be able to find a small, personally owned business that has them in stock though.
Hope your bearded dragon enjoys them. Cheers!