I do a lot of shopping for anime and manga stuff on - http://www.rightstuf.com/ - so I always recommend them if you're online buying.
Oh but just in case you didn't know Tokyopop was the licensed producer for Maid Sama and they only put out 8 of the 18 volumes. So yeah getting the whole thing may be like a dream for the future if someone else buys the rights or Tokyopop ever gets their head together again and starts printing books.
http://www.amazon.com/s/ref=sr_nr_n_13?fst=as%3Aoff&rh=n%3A4367%2Ck%3Amaid-sama&keywords=maid-sama&ie=UTF8&qid=1429392034&rnid=2941120011 Amazon link with the search already done
This is quite unrelated but does anyone knows any online stores that sells Maid-sama's manga in english scans? A friend of mine would like to collect the manga but couldn't find a good place to buy it... (=・ω・=)