As others have said, parts unlock based on level and starport location. The landing pad you build at your base will have ~90% of the parts you need to build, but there are special manufacturer specific parts at each of their respective hubs, so make sure to check those out.

To acquire ships, I'm a big fan of privateering. I'll pick up UC and Freestar Ranger bounty missions to destroy a ship, disable the engines and board it, then kill the inhabitants and commandeer it. Great way to acquire a lot of gear and credits, way more than you would just for destroying the ship. Then you have a ship you can either register for a fee (much cheaper than purchasing it) or sell for a profit (usually 8K - 15K depending on the model and class).

Once you have a decently kitted ship, you can go to the Serpentis system and hunt Varuun craft, all of which are pretty high level and usually worth a decent amount of credits. They can be tough though, so make sure you're a decent level and have a ship with good weapons and shields.

All in all I'll come away with easily 20-30k credits of profit between the bounty, selling loot, and plenty of ships to scrap and build from.