In the good old days, you’d have to go through a business broker, but there are now some great marketplaces you can find deals.

I bought my eCommerce store from Shopify Exchange, the advantage being you get to see all the actual revenue stats, not just claimed. It is a bit of a clunky system to communicate with the sellers.

Here are the best places I found, many having listings from $500

Shopify Exchange

A good amount of listings and gives great reliable data. Found only 1 in 5 sellers responded, though.
https://exchangemarketplace.com

Flippa

Lots of listings on different eCommerce platforms. Has a lot more thought put into it than Shopify Exchange
https://flippa.com

Empire Flippers

More focused on bigger deals. As I was looking for the smaller project didn’t find many that fit.
https://empireflippers.com

Micro Acquire

Great site with a variety of types of businesses. Well worth a look.
https://microacquire.com

Aussie Aquire

As it says on the tin, it is aimed at Australian-based businesses. Pretty new, so only a few listings.
https://www.aussieacquire.com.au

Hope you find a good one.