Can a simple DC power supply be made from a dimmer switch and a rectifier circuit?

No.

Conventional dimmer switches work by only turning on for a fraction of each half cycle. They don't produce a reduced sinusoidal voltage, they let through "time slices" of the full sized mains AC. These slices are at twice the mains frequency and for lighting this is too fast for the human eye to detect the flicker. Thus without smoothing and regulating, the rectified output would look like a succession of shark fins poking through the water. It would be "DC" but the only thing you would want to run off it would be a light bulb or a purely resistive heater.

Go online (Ebay , etc) and look for old Lambda, Heathkit, or Hewlett Packer power supplies. You should be able to find something for around 100 dollars that will suit your purpose. Even better get one that's broken and learn how to fix it.
What your purposing to try offers no isolation from the power line.

NO!

Your problem is that you don't know what to look for.

Does your power supply need to be variable, or fixed voltage?

You could buy a motor speed controller. I had a 90 vdc motor and got a nice controller for $29 years ago. It was 120 vac input.

You need to search online for a DC motor speed controller or a PWM controller with a 120 vac input.

There are plenty of low-cost motor speed controllers using battery power:
https://electricscooterparts.com/speedcontrollers36volt.html

Here's something from eBay, but it needs DC input. Just search for what you need. I don't have the time to find it for you.

https://www.ebay.com/p/12v-24v-Max-20a-PWM-DC-Motor-Stepless-Variable-Speed-Control-Controller-Switch/1462708855?iid=111936836128

Why don't you make your own. It will teach you a whole bunch of basic electronics. All you need is a transformer, rectifiers, opamps, zener or some sort of voltage reference, pass transistor, handful of resistors and capacitors, and a couple potentiometers (voltage & current control). With scrap material, you can build one for under $20.

Yes you can .Rectifier full bridge it will change the AC to DC which mean you can get DC out of a full bridge rectifier ,the amount electricity will be ACVx 1.44 = DCV .To increase or decrease or fixed DC voltage you need the Buck DC or Booster DC .Reminding Amount the AC voltage is important ,if you want a 12V DC ,first you need to transform (step down the AC) the AC to 12VAC then install rectifier ,which you will have 17.28 VDC ,then you can buck or boost the DCV to any designer voltage based on range of the buck or booster.

You are totally lost in electronic.
Dimmer is made of TRIAC circuit, output is in pulsing DC, has as high as 100Vdc. Dimmer controls the AC 120v power source sinewave phase angle to dim the device, not the voltage. As dimmer is set to minimum, the output voltage might still have 100Vdc.

No, Most dimmer switches chop the power into Pulsating DC which must be filtered. Look up[ circuits for the LM317 IC and build your own if you are up to it. Or try going to a surplus electronics store, there are many around which have used, obsolete and surplus power supplies. Surplus are usually left over equipment which was acquired from companies going out of business, bankrupt, etc. There are lots of deals and they will bargain for stuff.

Sounds like a good way to electrocute yourself.