Neither one of these are real close to Shelter Island.
Rosie's Calico Cupboard is at 71st on El Cajon Blvd.
Cozy Quilt Shop is in Rancho San Diego on Jamacha Rd.
http://www.rosiescalicocupboard.com
7151 El Cajon Blvd. San Diego 92115 (approx. 20 min.)
http://www.cozyquilt.com
2940 Jamacha Rd. El Cajon 92019 (approx. 30 min.)
Here are the links. You could do mapquest to get driving directions from your hotel.
Where will you be staying? Is this your first time in San Diego?