Affordable blackout curtains: tips, round-up, and best sources

I shared last summer how we keep our home cooler in the summer using blackout curtains. It means the AC doesn't have to work as hard, and we avoid hot spots near windows! Check out that post for the process/tips we use. I also shared in that post how I hacked some affordable blackout curtains to custom length and a higher-end look.
Well, I've been developing my opinions (and knowledge) about curtains a lot recently and have even more tips to share now, plus the best sources for affordable blackou...