I know some people who use beer for havdalah motzai pesach. It seems from halacha that wine should always be used even during the 9 days. Is there a source for the custom to use beer for havdalah motzai pesach?
Taame Haminhagim, footnote 70 to section 597 (page 249), says:
In the book Maase Rav of the gaon of Vilna, of blessed memory, item 185, the author writes that [the Vilna gaon] tried, on the night after [Pesach], to taste chametz, and also did not eat, after Pesach, matza one could fulfill the obligation [of matza] with, in order to distinguish performance of the mitzva: that one does it not for pleasure but because of the decree of the creator. This implies that there's some mitzva in showing that we do not need to eat [as on Pesach] any further. I've also heard that the rabbi of Sanz, of blessed memory, said havdala after Pesach on beer.
And in [Minhagim, by] the Maharil, laws of Pesach, page 19, the author writes, "On the night after Pesach, there was no chametz in the house of [?], and they were unconcerned and ate matza". See there.
Found it thanks to DoubleAA. The Rama 296:2 writes that is a custom to use beer on motzai pesach. However, on a regular motzai Shabbas he writes that pagum (someone drank from it) wine is better than beer.