Most major retailers can, and do, stock at least 4x5 Portra. Perhaps your local mom n' pop camera stores might not, but major players like Samy's, B&H, Adorama, etc certainly do. At least here in the USA.
Kept in frozen conditions, color film can last a good long while. I'm talking well over a decade. I shot some 1994 Vericolor VPSIII a month or so back, and I had literally zero fogging of the film. However, I purchased it from a photographer some years back who was dismantling his studio, and had a freezer full of film, mostly 4x5. All purchased new, I got it for pennies on the dollar. But that's me. I don't see why more modern films will/would be any different.
*However: I would NOT recommend re-freezing open packages of film once the factory interior foil bags have been opened. They protect against atmospheric contamination and humidity. If I remove film from my film-only chest freezer, and do not use the entire pack, I will leave the now-open package, closed up/boxed in the top of an interior closet in the house, where it's cool and dry. I have been working from opened boxes of film kept in this storage state that were were first opened up over 2 years ago(these are 50sht boxes mind you, and I don't shoot a huge amount). I have not seen any issues stemming from this, whatsoever.
Film is quite robust, but still requires you to not abuse it. That means not leaving it for months on end in the trunk/boot of your black car, if you live in the middle of the desert
. All joking aside, I've left roll film and loaded 4x5/5x7/8x10 holders in the boot of my car for weeks on end, and have had no ill effects on the film from heat or other maladies relating to storage conditions.
-Dan
