YouTube has tons of vids available for making spaceships from trash. If you need ideas for bits for greeblies, go check out Scratch Bashing. He has several "Monthly Haul" vids where he showcases the random trash he's collected and what he thinks can be done with them.

Also check out Bill Making Stuff.

You can also design your own greeblies using EVA foam, a blade, and a heat gun. Bit time consuming but EVA is probably cheaper than sheet styrene

EDIT: Now that I'm at my computer, here's some links:

https://www.snapships.com/product-category/products/

https://www.amazon.com/Exclusive-Expanse-Rocinante-Replica-Stores/dp/B079Z2JMMX

I've personally seen these used as some good, cheap sources for sci-fi greeblies:

https://www.amazon.com/SCS-Direct-Fantasy-Sci-fi-Figures/dp/B0746T9X9C

https://www.amazon.com/ArtCreativity-Spaceship-Children-Assorted-Galactic/dp/B0932HJ28F