Does any manufacturer offer pipe sections longer than 36" or even custom sized up to 6 feet?
Any brands of good flashings still made and galvanized in the US?
Here's a link to a commercial website that sells most of the major brands. They have the catalogs/brochures in pdf format on their website. The longest I saw was 48" but only looked at one mfg.
Don't know about the "galvanized in US" question.
Here is a link that might be useful: venting pipe