Not sure what you mean by "heirloom."
Berkeley Bowl sells popcorn on the cob. It is about $1.50 for a bundle of three. They are not marked "popcorn" as such -- I'm sure most people just buy them for decoration
I bought the small red cobs. You put one of these (the whole cob, without the husks) in a small paper bag and microwave it until it pops.
Be careful -- if you overcook it, it will catch on fire-- very bad.
2020 Oregon St, Berkeley, CA 94703
Rancho Gordo beans carries Red Crimson popcorn. Their products are available in the Bay Area, though not in WC. Everything they offer is amazing. Their shipping is quite reasonable at $8 flat rate per order in that you can stock up on their truly incredible beans and hominy and just pay a flat shipping free.
1924 Yajome St, Napa, CA 94559