You can't use PS1 accessories on the PS3? I'm really not sure, I don't own one. But the PS2 can use PS1 accessories, and I know you can play PS1 games on the PS3.
If you can, then just use the Namco GunCon controller. It works great and was created for the Time Crisis games. The GunCon2 is even better, as it has a d-pad right on the back of the gun (which works great for Resident Evil: Dead Aim).