How to get older versions of PS

I own an old license for Photoshop Elements 10 and am looking for a newer version to buy. I have an older iMac that doesn't support the latest version. I was able to download a trial version of 15 and wanted to purchase but the Adobe store said it was not longer available to purchase. Why have a trail if you can't buy? Anyone know where I can get a newer one? Thanks. I'm running High Sierra OS.