Here's the IFIXIT [guide|18695|MacBook Pro 13" Retina Display Late 2013 Teardown] so you get an idea what your system look like opened up.
Presently Apple is the only source for the M.2 PCIe blade type SSD units used in the newest MacBook Pro 13" systems.
If you have an early '13 or older model then you can go here: [http://eshop.macsales.com/shop/SSD/OWC/Air-Retina/Apple-MacBook-Pro-Retina-2012-Drive-Internal-Flash|OWC - SSD upgrades]