The nearest is Locarno (LSZL), some 30km from Lugano
Homepage of the Gruppo Volo a Vela Ticino: http://www.gvvt.ch/

Somewhat farther away is Ambri (LSPM)
Homepage of the Gruppo Volo a vela Leventina: http://www.gvvl.ch/

Note that both operations are clubs, so it's not sure that you'll find
an instructor on working days. On the weekend it should be no problem to
fly with an instructor if you call them early enough, but I doubt that
it will be possible to rent a plane for solo flight.