Well you can try eBay, but I know of a FS98 MS DOS version in our local habitat store, which I almost purchased. If you want FSX, then eBay would be your best bet. GameStop does not handle it, only play station, and the like type. Look in thrift stores as well, since the employees do not know mostly what it is, it will be in the CD area.
If FSX is your goal, then make sure you get the two service packs issued for it. Just search internet for them. Process is install game, register it with code in box, install SP1, then restart and SP2. You also might need to run KB-928080 so machine can run it. Search for installing FSX here, I posted a long sequence so it will run. Lots of websites have tons of free planes and scenery for FSX and it’s still going strong. I had it on PC, but rebuilt it and have not reinstalled it. Happy Flying.