Here's more detailed info for Vintage Instruments... this is closest to where you'll be staying...and will probably have more mandos than the others... although they will all be vintage... lots of old Gibsons and Martins and various bowls. #Definitely call first... they don't always actively staff both locations for walk-in business... but if they know you're coming, they'll arrange to be there with you.
1609 Pine Street
Philadelphia, PA 19103
Guitar Shop Phone: 215-545-1000
1529 Pine Street
Philadelphia PA 19102
Main Office Phone: 215-545-1100
HOURS: Monday-Friday 10am to 5pm
Closed for SUMMER VACATION July 24 to August 6
After that, I'd say Todaro's. Especially if you're interested in bowlbacks. Joe Todaro has been involved with mandolin orchestra, and has a bowlback that he is having produced for him to sell, with a one-piece carved bowl. I haven't gotten there myself, but it's intriguing. He also carries Weber mandolins. Also... fretted stringed instruments from many other places. here's a link: link
Otter OM #1
Brian Dean OM #32
Old Wave Mandola #372
Phoenix Neoclassical #256
If you're gonna walk on thin ice, you might as well dance!