In Siena there are 3 official shops "TuttoWind" here they are:
FUSI FORNITURE UFFICIO
PIAZZA LA LIZZA 13/17 - TEL 0577/41186
STUDIO DI INFORMATICA DELL'ISTITUTO Tutto Wind
P.ZZA FABIO BARGAGLI PETRUCCI 16/18 - TEL 0577/288045
TELECOM S.R.L. Tutto Wind
STRADA MASSETANA ROMANA 50/A - TEL 0577/271661
At Malpensa airport there is just a Vodafone shop and a "rent a phone" shop, so I think you won't find anything useful there.
Anyway if you need a Wind card in advance we can get it for you without the usual problems with "codice fiscale" and registration; take a look
HERE.