Dog Lover,
UMD movies are in very limited production and are hard to find in stores. Due to poor sales, most of the major production companies began to cut back on their sales/marketing of the UMD movies. You can go to sites like Amazon.com, SonyStyle.com, or PSPWorld.com to find the latest releases.

There are also ways to convert your own movies/DVDs to formats that you can play on your PSP. Check out PSPVideo9 to learn more.

I haven't bought a UMD for it in a while. I didn't know they were scarce.

hmm. I didn't know they were scarce either.
they are usually available at Best Buy though.
and i have never had a problem finding them.
I recently bought Superbad for UMD in December, so maybe the sales have gone down since then?