I haven't personally run into this as a problem--I've rarely had any instances where a combat lasted long enough for the wizards to run out of stress completely, and when they have, they tend to invoke things or take consequences--remember, at 5, a wizard has an extra mental mild consequence, so they've got a total of 6 spells.
Generally, I allow them to invoke something (either a scene aspect, one of their consequences or aspects) to eat up to 2 stress, so they can cast a spell equal to their second stress box in power using the aspect as the 'source'. This is also a good way to look at, say, sponsored magics--taking debt for the same reason.
I also wrote up a little while ago a way to extend magical attacks similar to how you can extend blocks.
But all of that aside? The RAW key to a wizard's longevity is enchanted items. I have one wizard NPC who could easily go eight or nine rounds throwing off Weapon:5 and 6 single attacks, and a Weapon:4 zone attack, with an armored coat; said character, when prepared, will pretty much never have to worry about running out of stress to cast spells with.