Where do people level alt jobs now?

[Rehayem]

Get yourself a Squadron Battle Manual (you get these by sending your GC squadron on Infilitrate and Rescue weekly mission, if they succeed). Add the FC exp buff to it, and queue to dungeons that are on your level. If you still have a hunting log to clear, that's also a nice source of exp. Do some fates while you wait for dungeon queue to pop. Oh and do your roulettes. Queue for Frontlines as your archer, then you can switch jobs inside, that whole EXP is gonna go towards your archer.

[Iscah]

Get yourself a Squadron Battle Manual. Add the FC exp buff to it...

Don't the squadron manuals all override FC buffs with the same effect?

[Evergrey]

Don't the squadron manuals all override FC buffs with the same effect?

"Grand Company-issue literature outlining basic combat techniques.

Increases EXP earned by defeating enemies in the field.
EXP Bonus: +15% Duration: 120m
Overwrites company actions with similar effects. "

Indeed it does.
I do however believe that, if I remember correctly, food buffs still works.
It's slim, like 3% or so. But hey, it's at least something.

Raisins is a cheap food one can get from NPCs.
I tend to stock up on them each expansion for my leveling journey.