There are tons of Block Libraries available on the Web. I always just built up my own based on whatever Industry/area I was working in. Is there a specific 'thing' you are looking for?
Van is right but they always want you to sign up, allow cookies, and try to charge you. The ones I use I've gotten from ETC, Clancy, SECOA, H&H (some of the best), and the like. IMHO googling for cad blocks isn't usually worth it.
Truss blocks should be available through most manufacturers is not the website then a quick email should get you blocks. Tables and room planning, again there are tons of free CADblock sites out there you may simply have to glean what you can from the free sites then delete all the crap they try to sell you. I wish I could point you in one direction and say "there's what you need!"
In fact, I agree and that is mostly what I've done. Sometimes with a new product I'll use the manufacturer's. I'll concede I've used a lot of ETC's Prodigycad files recently.