I'm a big fan of Argento's work and have spent years putting together a daybill collection of his films.
The rarity of these titles varies, but not with age. The hardest to find titles (in my opinion) are 'Tenebre' and especially 'Deep Red'. For reasons unknown to me, the daybill for 'Deep Red' is about a 9 / 10 on the unicorn scale - extremely rare.
'Suspiria' and 'Inferno' appear regularly in various auctions and his original animal trilogy - 'Bird with the Crystal Plumage', 'Cat o' Nine Tails' and 'Four Flies on Grey Velvet' - seem to fit somewhere in the middle. 'Flies' seems to pop up more often than the other two.
Argento's work seems to have received much greater cinematic release than other European horror / thriller directors of the time, probably a reflection on his standing in the field in the 1970s.
My collection stops with 'Tenebre' which (I believe) was released locally in 1983. He released 'Phenomena / Creepers' in 1985 and 'Opera / Terror at the Opera' in 1987.
So far, I'm only aware of these films receiving VHS releases, but I'd love to hear if anyone is aware of a daybill for either of these titles.