Don't know how "special" the O-rings are, but would imagine they'd have to be high heat. Maybe someone has a size they can post. Maybe take the pipe with you to auto parts or such, and with knowing outside dia. of the exhaust you could find something. Ya, you could order some from FMF (or anybody) but might be fall before it clears costoms, and cost ya 10x as much.
As to the PVC caps, I just took a boot to the hardware store and found the best fitting piece I could. Was actually grey, might have been in the electrical conduit section. Leastwise here water lines are white and elec are grey. I should have cut them a bit for length for a cleaner look. I did this because the boost bottle was starting to crack the boots, not enough to need new, but still.