What is an economical way to supply DC power for small projects?
The cheapest, most universal way is to use a USB connector for 5V. I’ve taken to using this method instead of buying barrel-plug power supplies. It has saved me a lot of time and effort. You can find adapters to USB power from just about anything you might find in the home: batteries, solar chargers, 24VAC (think sprinklers, doorbells, theromostats), line voltage, your R-Pi, your desktop PC, etc.

For higher voltages (9, 12V) at moderate current (2-3A) look to the barrel plug type. CUI makes both the barrel receptacle and wall-wart supply in various output voltages. Phihong is another popular brand for this kind of power supply. You can find these on Digi-Key, Mouser, etc.

For even higher voltages (24, 48V) and currents (3A or more), consider a screw terminal block. A popular quality brand of high-power supply for these is MeanWell. Jameco and others sell these.

Some really high power projects can use a PC power supply and use the ‘GPU’ 12V power connector, or native ATX. It’s hard to beat an ATX power supply in terms of availability and watt/$. You can also find line-to-GPU power as a separate unit - mining rigs use this type sometimes.