Basically you double your mod capacity(doesn't matter when you use it, you'll always have twice the level of mod spots) when you stick an Orokin Catalyst in a piece of gear(Reactors for frames). Add in formas and you get upwards of 200 mod spots for your high-end gear effectively.RJ 17 said:snip
Edit: Also, if you get any Aura mods, level it up and stick in a warframe(though obviously try to get something you think will be useful) and you get that many more mod slots for however much it says it "costs" to stick in. Also I think something similar happens with melee stance mods. I forget.
Also make sure that you fully grind out at least one planet other than Earth or Mercury. You do that, you can start getting nightmare missions and mods(and with the early planets open they become piss easy runs with the exception of Defense-type missions) which go for pretty good plat in trading if you farm enough of a certain one to get it to max. Unless you want them yourself(though with your setup you're going for I don't think there's alot of them that you actually want).
Edit2: Realized I didn't tell you how you get them. Watch the invasions mostly, you can get blueprints(though they're one of the things you get in the game that the blueprints are one use only so you have to go get another one). Also there's an event going on that you can get one easy if you like spy missions(and have up to like Jupiter and Saturn unlocked). Also the alerts sometimes drop them, and they're also dropped in like Tower2+ survival I think. They're rare, but oh so worth it.