There's loads of them out there if you Google the phrase "dual twin sim 6310i", e.g. [broken link removed]
I used to use one on another phone that needed you to switch the phone on and off in order to change SIM. Some of the newer ones seem to be menu driven. One thing to note though is that if the phone is locked to a particular network you will have to use 2 SIMs from that network or else get the phone unlocked.