Errr...don't think that is ever going to happen Sorry.
No "off the shelf" package will ever come with source code, maybe the odd exception. The cost to make these packages costs hundreds of thousands of pounds...I very much doubt that a company will just give you the source code.
They will also, no doubt, encrypt their data so that you can't "add on" other projects for customisation as they won't be able to read/write the data...

If you went to a software house, which is where I worked for my previous job, and asked them to write you an accountancy package then this would cost easily 10K + Then, in this case, you may get the source code, but only if it's in the contract. Bespoke software is VERY expensive...

Regards,

Woka