UMD movies are completely irrelevant in multiple ways. They were tried as a cheap way to get a little more cash, and when they weren't really successful, they faded into obscurity. A lot of systems have failed experiments. That's nothing new. As for M2 cards, nobody really needed one which is why nobody really stocked them. They were only needed for the Go which came absolutely loaded with memory to begin with. The average owner of the Go didn't need a M2, thus, nobody carried them. As a store, you don't order stock that you know for a fact won't sell. You are basically saying "hey, stock shit that won't sell so that I won't be inconvenienced". Try a little less self-entitlement and a little more logic.
As for not being able to locally purchase compatible memory sticks and batteries, lol. You realize the PSP died in the US several years ago, right? Just because it was and is still alive in Japan doesn't mean that US retailers were going to keep on top of the miscellaneous PSP supplies. It was smarter for the average store to simply leave those types of things to online shops. Hell, even when the PSP was at its peak, I'm pretty confident it was easier to use an online shop to get what you needed. Really though, complaining that stores don't locally carry stuff for a dead system is like complaining because you can't go to any electronics retailer and still grab a fresh new PS2 controller. When something stops being relevant, stores stop stocking stuff for it. Expecting them to bend your miscellaneous whims because you entered the game late is just silly. Especially in this age of devices being built with rechargeable batteries and a dependence on external memory, it should be fairly obvious that they come with an officially limited lifespan.