you could download them, but that would be illegal. therefore, i don't recommend doing it. not only is it against the law and bad karma for ya, no one here will give you support on a downloaded game (for the above reasons, and also because downloaded games are notorious for being sub par quality).
just buy a new CD rom drive for 20 bucks to make your own ISOs or play off the CDs, or go to the used game store and buy a PSX for like 20 bucks.
i think i need to take the time right now to explain to you waht a tech demo is.
a tech demo is NOT a game demo. they arent the CDs you get with PSM magazine every month. tech demos are homebrew games, programmed by ordinary people, and offered on the internet for free. they are usually very simple and/or just showing off a technical principle. hardly ever are they games that you'd actually play all the time, the most complex one i ever played was a simple version of Legend of Zelda 1.