Here in the USA, you can purchase signed copies of my books at the following shops.  Nearly all of them will happily ship to you!   

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California Barnes & Nobles 
They have the B&N exclusive purple edition of Family of Liars, plus We Were Liars and in most cases, Genuine Fraud and maybe Again Again

BN San Bruno
BN San Mateo
BN Dublin
BN El Cerrito
BN Beryville
BN Marina del Rey
BN Grove
BN Studio City
BN Americana

California Indies
Look here for the regular blue Family of Liars, plus We Were Liars, Genuine Fraud and maybe Again Again.  Plus they might have Whistle: A New Gotham City Hero, and if noted below, they have my Emily Jenkins books signed as well. 

West Portal Bookshop
Keplers
Linden Tree Bookstore
Hicklebees (has lots of Emily Jenkins books)
Books Inc

New York City Indies

The Center for Fiction (Family of Liars only)
Greenlight Bookstore (all recent Lockhart books)
Community Bookstore (Family of Liars only)
Powerhouse on 8th (Family of Liars only)
McNally Jackson (Family of Liars only)
Books of Wonder (all recent Lockhart books including Whistle, plus Upside-Down Magic)
The Strand (all recent Lockhart books)

New York City Barnes and Nobles 
They have the B&N exclusive purple edition of Family of Liars, plus We Were Liars and in most cases, Genuine Fraud and maybe Again Again

BN Court Street
BN Park Slope
BN Union Sq
BN Tribeca

And in Connecticut:

RJ Julia