I grew up similar to the OP, with a Dad who had minimal tools and didn't believe in buying metric tools.

I picked up a large collection of tools that now include everything from wrenches and sockets to mighty vacs, impact screwdrivers, torque wrenches, a bunch of specialty VW tools, and even a monstrous cherry picker.

Yardsales- tons of opportunities here, especially when you find estate sales

Craigslist- A while back I found an ad on craigslist where a guy was selling a nice rolling box for 200. He took a job with the border patrol and needed to move ASAP. I went and picked it up, and his wife emptied all the tools out of it. He called me back a couple hours later and asked me to come back- the tools were supposed to be included with the box! I ended up with a full set of sae and metric SK sockets and wrenches from that one, then sold the box for 150 a couple years later.

Coworkers- When I started working in shops I noticed that a lot of the older techs had huge collections of broken Crafstman tools that they had long ago replaced with Snap On, etc and never got around to exchanging. I bought a whole mess of them for pennies on the dollar, and exchanged them at Sears.

I've never had much luck with pawnshops. Their prices are overinflated, and they usually want to sell the whole package box and all.