I am new to psn.
As others have said, no, you can no longer buy the original PlayStation in stores (brand new.)
You can try to find a used one at any local game shops in your area (that sell used / old games & consoles.)
You can try Craigslist and eBay. Depending on the model and if it is complete-in-box, they are pretty cheap. ($2 - $50 bucks)
Or if you're a collector and looking to find an original factory sealed PS1 -- you're looking at asking prices in the hundreds.
You cannot buy them anymore at standard stores. You can however find them at several retro game stores, or online.
However, remember that while only some PS3 units are able to play PS2 games, ALL Ps3 units can play PS1 games.
Also note that PS2 can also play them, and while the PS2 is discontinued as of December 31st 2012, they can be found new sometimes. I seen a new one for $99 at toys R us just last week infact.