You can't put hands on a jet, wraith can have turrets though although there aren't that many in the game to pick from to get good ones.
Okay, here's a list of great parts to scrounge:
Computer 4 (200 capacity) on Luna II: 1t, 4 slots
Computer 2 (100 capacity) on Vadel, Ramuh: .5t, 2 slots
PAR-6 heavy particle on Green Pixie (!): 1t, 3 slots (versus normal 3t, you get only one per green pixie so saving these up takes a long time or a lot of money)
Sparkle Missiles x40 on Blue, Green Pixie (!): 1t, 12 slots (overload and swarm is nifty).
AM Beamgun on Neko: 1.5t, 2 slots (smallest antimissile system with BV that uses battery energy)
Piloting Bonus Software, any Savin: 70ZeG needed, gives +2 pilot mecha
Battery 2 on Ovaknight: 0.5t 2 slots (really, really useful for later fights or slaughtering multiple encounters at once with all those underweight PAR-6s you get from Green Pixies)
Heavy Actuator 5 on Ovaknight: 0.5t 5 slots, each of these increases base walk speed by 5 and they also increase melee damage... which can be FUN if you are really, really good at mecha weapons/fighting and have a super-fast mech.
You can't overload more than five slots per module anymore though so you can't stack arcjet, flight, hover and overcharger 9s in the same limb like you (almost?) could in GH1; small mecha just aren't as fun as they used to be, so I don't know if there is still a hard-ceiling on speed like there used to be in GH1... I guess the HAs are kind of something to stick in your parts stockpile in case you feel like saving and doing something silly instead of loading on your main battle vehicle.
There aren't enough bomb or other area-effect weapons besides swarm missile packs (which are the single easiest weapon to defend against in the game) and most of the best ones seem to come from special-mod mecha--I have a "Fireworks Spectacular" that weighs 14.5t and is a rifle with BV:3, BLAST, SWARM and BURN attributes. I *wish* I could use this in my arena games but my sort-of Green Pixies just get knocked around by the recoil and only manage to hit one target with it no matter how high my gunnery is.
....Is there any way to make a mech with a head-cockpit, reflex system and high-performance? It seems to be the only way to even start maxing out what you can put on your battroid before your MV/TR goes through the floor straight into incompetence.