Because she was tired of being a glorified backup dancer.And being in the shadow of Nicole all the time.Jessica even said Nicole got to control how much camera time each of the dolls got in Videos.
Here is some interview she did after leaving:
http://www.youtube.com/watch?v=rwBHKxENIvU (E News)
http://www.youtube.com/watch?v=kTSoVZfnaEM (Abc News)
She is working on a solo album yes.
You can also follow her on twitter if you want
online, from magazines and etc
http://pussycatdolls.shop.bravadousa.com/Dept.aspx?cp=15710_15867 ( PCD's online shop) its just of PCD not just Jessica)
Just posters of Jessica are hard to find cuz they are either of just PCD as a whole or mostly Nicole. But i would suggest trying magazines or ebay,amazon or PCD's onine shop i gave you