score:7
You can find a TIM point in almost all shopping centers, or phone shops because they are retailers in Italy. You can use the "store search" on the website (You can also shop Sim Card on Online Store) 
https://www.tim.it/trova-negozio?_DARGS=/browse/trovaNegozio.jsp#
The better simcard in Italy are Vodafone, Tim and Wind. I will not recommend '3'.
UPDATE:
Where to buy a TIM simcard in Malpensa airport?
Terminal 1 on Malpensa: 
Terminal 2 on Malpensa: 
UNIEURO Tel: 0274868169 here 
All days: 06:00 - 21:00 
Saturday: 06:00 - 20:00 
WIND 
All days: 05:00 - 21:00 
Saturday: 05:00 - 18:00 
Near Malpensa: 
Tim Tel: 0331701115 e-mail: [email protected] here 
Piazza della Libertà, 1, 21013 Gallarate (VA) (15 minutes car/40 minutes bus)
Monday 15–19 
Tuesday/Friday 09:30–13 15:30–19 
Saturday 09–13 15–19 
Sunday Close 
Ram S.r.l. (Vodafone) here 
Piazza San Lorenzo 9, 21013 Gallarate (VA) (15 minutes car)
3Store tel: 0331250205 
Via Pastrengo, 1 Somma Lombardo (VA) - 21019 (15 minutes car)
 Check in the Mobile App of Milano Malpensa. And If you need something, tell me as well.