Just to expand on this a little:
T1 BPOs are seeded in NPC stations, and these can create BPCs easily through standard copying.
T2 BPOs technically exist, but no more will be seeded and the originals are insanely expensive (they pop up here now and again the in market place). Generally, T2 items are built through T2 BPCs that are created through Invention.
There are selections of specialized meta modules that players can build using BPCs that can be found through either exploration or running things like COSMOS missions. Most lower-tier meta modules do not have any kind of BP available and cannot be built by players.
Faction module BPCs can be bought through LP stores.
Faction ship BPCs can be bought through LP stores, but there are some other options as well. Pirate ship BPCs can be found through certain rare drops in DED combat sites (SoE ships are a little more convoluted, as you can get Nexus chips that can be redeemed in SoE LP stores for SoE ship BPCs) or may be given out in COSMOS or Epic Arc missions.
I think that covers it, but I might be missing something in there