My only suggestion would be to find a BMC dealer and ask. I have had pretty poor luck finding small parts for my Time Machines otherwise.
I'm in the UK where Evans Cycles seem to be the main dealer and I've just arranged to get the part from them (£4 for the part plus £5 processing/postage). It looks like the previous owner has replaced the standard pipe with a flexible one, I can only assume he tried to widen the hole for some reason.
I've just ordered a generic 90 degree pipe from eBay for £2 as the official one costs £15. Do you think this will be ok, or should I stump up for the official one?