FYI for those who might be looking - I found 2 places - one is Sanitätshaus St. Johann ( http://www.rehamed.ch/rollstuhle_standard.html
) opposite the University Hospital (Childrens) and the other was rehamobil (http://www.rehamobil.ch/kontakt
) which is near Schutzenmattstrasse bus stop (about 250-300m). The first had only childrens ones in stock and directed me to the second one. All were very helpful.
Rehamobil charged: 70chf for rental for a month, 30 admin/cleaning fee and 200 deposit. Because I wanted it for a short time they have a pro rata reduction - and for 5-15 days you get a 40% money back. Overall I paid 42+30 CHF.