Not sure if I'm missing something or it's a bug, but the only way I can get supplies (in particular food) is convoys!?
Where are all the steal the plane and helo missions?
I'm only tier 20 and want to upgrade various weapons but have no food, convoy after convoy gets monotonous.
Not quite. You progress your Level from 1 to 30 and some skills unlock with your level. Once you reach Level 30 you are given the option to switch Tier mode on, which then progresses from T50 down to T1 with increasing difficulty, your Level staying at its maxed out level of 30 (Fallen Ghosts aside).
You can turn Tier mode off but you cannot go backwards - there is a very good argument to suggest you should be able to - and at certain Tiers you get unlocks but also restrictions on which difficulty you can choose.