Unless for learning or debugging purposes, you won't need the SF source code right now to continue the development from where it stopped. Of course, if you guys have a current subscription, you can just download the beta installer (latest version) and then go to the "My Account" area and download the source code for your convinience. For that, you will probably need to ask the company that bought SF for a valid username and password.
It is really important that you get to know the structure in which SF bases its functionality. And having I said that, you do need access to the SF database, which probably already exists there somewhere on your company installation. You will want to have that to continue the work from now on.
First of all, you will want to get the StrataFrame database from the people who mapped this project. This is where all of the mappings reside for the BOs, etc. Otherwise you will have to recreate the mappings.
Once you get this, just load the new SF update and it will update the structures. If you cannot get the original StrataFrame database, then you will still need to load the update and allow the StrataFrame database to be created, but at this point, you will have to manually map the BOs to their source manually.