apart from them producing higher frequencies, components speakers are preferred because of the multiple mounting options for the tweeters. Meaning the vocals are faced right at you rather then bouncing off your legs, seats etc.
Yes they are available without components. the 2 easiest models you will find are the PIONEER TS-S20 (cost around 3k) and SONY XPLOD H-20S (cost around 2.5k). Id stay away from chinese stuff even if theyr considered very good.
The can be fitted directly to a PRE. these tweeters have an inbuilt crossover so you can just run them off the speaker cahnnels on a pre.
However just like a midbass driver, these also need alot of power (40rms+), while almost all pre's have an output of 22rms per channel, so if u get an amp, not only will you have more options of frequency tunings but also your tweets wont be underpowered.