Not sure about pro circuit, but the Toomey pipe and flange uses a #223 high temp (red) silicone O-ring.
Which fits what i believe is a standard header flange. As to whether or not the pro circuit pipe has a slot machined for the oring to fit in, I cannot say.
If you need the O-ring oringsandmore on ebay (also at oringsandmore.com) has a 15 pack for around 50 cents a piece shipped.
I ordered from there and 2 or 3 days later the orings showed up, quality looked as good as any and they fit just right.
Pretty good deal if you ask me.