Are these possible places to find lost media?

Yeah that's not how it works.

Just because a product has been 'discontinued' doesn't mean stores like Walmart, Target, etc. will stop selling it. For a non-media related example, look at the concept of shelfwarming when it comes to toy collecting, the term refers to toys that just aren't selling and are (metaphorically) just sitting and warming store shelves. To give a personal example, my local Walmart was still selling Rogue One: A Star Wars Story Black Series figures (which were released in 2016) in 2018. Two years after release and they were still warming the pegs (although they eventually discounted them to $5 a pop to try and get rid of them, not ashamed to admit I bought one at that price)

Anyways, if a product at a major retailer genuinely doesn't sell (whether it's discontinued or not), one of five things may happen:
- The product will end up sold at a discount to one of those discount outlet stores (think TJ Maxx, Ollie's, Tuesday Morning), where they will try and sell it for a much cheaper price
- It ends up returned to the original manufacturer and the retailer gets a refund on the product (if applicable and from what I know, only the actual major retailers can do this shit because they have the leverage power over the manufacturers)
- It does temporarily end up stored in a warehouse, where it will eventually be sold in a liquidation auction
- In some cases, it may end up stored somewhere because the contract the retailer has with the manufacturer prevents the product from being destroyed, resold, or sold after a certain date.
- The retailer will literally just discard or otherwise destroy the product

But this is all talking about discontinued products that actually made it onto store shelves, for stuff that never actually made it onto shelves, there is a chance that it could still be rotting away in a warehouse somewhere. It's one of the sources of new old stock, after all.

But you still shouldn't think of this as a potential source for lost or rare media, not saying it couldn't happen - there could certainly be some old warehouse or storage facility somewhere with a stock of sealed and unsold VHS copies of that super rare movie from the 1980s people are trying to find that never shows up on auction sites and has never been uploaded onto the internet or something. But with the sheer number of these facilities that exist around the world, the chances of you (general you here) going to a warehouse or storage facility auction and finding some lost/rare media is basically slim to none.

Big-box retailers in my area will sometimes dump inventory at our Goodwill. Things like phone chargers, games and trading card tins, still sealed with their price stickers; even in larger quantities.

Has anyone had luck finding interesting things on used computers and hard-drives? I imagine a lot of them are wiped prior to sale by whoever is selling them, but less professional vendors may not.

Maybe Craigslist, but would you really want to start buying up random used computers on the off chance that they're not password protected AND have copies of rare lost media on their hard drives?