They are a little rarer than weapon schematics, but this is mainly because you can't relogski them at irregular markers.
So its a matter of finding the right spot to farm them. e.g. if you orbital pirouette military settlements you can gather them pretty effectively (loot, launch, SC, drop SC, come back down, repeat... I know... real engaging gameplay yes?
The trick is to not just focus on them, like a lot of stuff.
Large high tech bases are a great source for suit schematics but if you ignore the rest of the base you'll just wind up needing to focus on gas and other assets later on (and data, of course, which you can get a fair amount of, dependent on RNG).
I've found I can basically get by with three activities:
1) Do missions for the engineering data (not the unlocks, the data needed for the modules) and loot as I can, where appropriate
2) Run through large extraction and high tech bases, stripping the whole base (and stealing the PR at the end when I need PRs)
3) Do the odd mission to keep MIs topped up (sometimes you see them for 5), and occasionally for gas or PRs
I did do the IM and Impact relogs early on, admittedly. But looking back, I don't think that would have been as necesasry as I initially thought. I've not needed to do that much since. Just one IM focused session for some weapon schematics when I needed them, maybe. Although still, not stricly necessary. The bottleneck was still the gas, after that. So I may have been more efficient focusing on a high tech base instead.
The only consistently disruptive activity for all of this is the high data demands for unlocking. I'm at 26/40 OPs now, so clearly not ahead of the curve on that. And the 15 SDPs would probably still be locked up for me even now, had I not used the IMs for that.