In all seriousness (what? seriousness? in a thread asking about Fett's groceries?), I'd think military rations would be good enough for Fett. Bland and pasty, sure, but all the nutrients a body needs. Cold and efficient, seems to fit well. I'm sure whatever company supplies the Empire makes excess...it's not like anyone else is going to buy any. Or maybe he just stocks up at docking bays? They've got to have places for spacers to resupply.
As for the weapons...the regular guns are available just about anywhere. Armor and armor-related weapons? I'm sure Fett's handled his share of antique dealers. Mandalorian battle armor must be popular with them.
GPI: Fondly regard crustacean