For small panels, check sites already suggested by others. If you want to experiment with larger panels, craigslist or similar have good deals. Used panels or new leftovers from a pallet.

I've paid $130ish each for 4 panels 280w-315w. And since it's local, you don't get dinged for shipping.

For hobby/experiment use decomissioned panels from upgaded farms, sometimes you can get two years old panels that were swapped for more efficient ones for a fraction of the original price.

For setup with batteries there is (depending slightly on local electricy cost) no long time break-even - the battery deprecation due to discharge cycle costs more than electricity from the utility.

I'd look for solar panels intended for camping.

Probably eBay or Amazon is the easiest place to look for cheap gear. Hobbyist electronic stores might be a good source too depending on the country (here in Australia Jaycar has some fairly good value panels and PWM solar charge controllers).

Unless you don't have the space there's no reason to go smaller than a 100W panel. You can find them used for $50 or less. For smaller panels the cost per watt goes up enough that it ends up costing the same for less output.

If you're in the UK, Pimoroni (https://shop.pimoroni.com/?q=solar) has some good options as well.

Adafruit has a variety of them.

https://www.adafruit.com/search?q=solar%20panel

Sparkfun is a great goto place for electronic stuff like this. However, I’m sure there’s more specialized solar panel sources.

Https://sparkfun.com