I think the PT-30 has a 67mm screw type base like most of the Olympus housings. I recommend the Inon UWL-100. I've used one for a few years with an Oly 5050 and PT-15 housing.
I've had good luck getting stuff for a good price from a company in Japan:
UWdigitalcamera.com
It's about $245+shipping
Ryan has it for $329+shipping