Availability
Planetary v1 should be available at your local comic book shop, Amazon.com, or other booksellers. It’s been reprinted in at least three formats, a trade paperback collection (cover shown above), an Absolute edition that collects the first half of the series and, most recently, a massive Omnibus edition that collects the entire series.
Accessibility
Some violence and blood, harsh language and some adult themes. Probably suitable for teens.
The series requires no real knowledge of anything outside of itself, although being familiar with the Wildstorm Universe (particularly Ellis’ other book at the time, The Authority) would help, and there may be literary or historical references you won’t get if you’re not familiar with them. It’s one of those books that gives back to you at the level you put into it, although it’s not as deep as (say) Moore’s League of Extraordinary Gentlemen.
Verdict
One of my favorite comic book series of all time. Highest possible recommendation.