There's this company. Very expensive and they're in China. With the long startup times of Blackberries it's probably easier just getting a second phone.
Dual sim card for mobile phone BlackBerry 9530-Storm
You can also try Ebay, although the Storm isn't mentioned in here:
GHOST DUAL SIM FOR BLACKBERRY 8100 8120 8130 8820 8830 - eBay (item 170372725270 end time Oct-15-09 06:14:30 PDT)
Someone reviewed an adapter from the Chinese company in an older Blackberry. Can't remember if it was here or another forum.