The source from whom he was arranging 15k is not able to fund/lend him, so our realtor has suggested my friend to take 15k from me and repay me in monthly installments which will keep the partnership 50-50.

As long as you have a source to build up more capital for the next purchase I would say you have chose a good way to get started. 

We feel we have exceeded our investment potential through friends and family and would like to start searching for other sources of funding such are touching base with funds, family offices or sophisticated investors.

Considering your income source is a key factor one needs to understand and take into account when looking to get a loan... 

Good cashflowing properties can still be found you just may have to get creative or find a different way to source properties.

However, be sure the source of your info is good.

You example needs to be evaluated using all sources of return. 

Or could make an educated guess (with link/ sources of course ;-)). 

But you are correct that you will need a source of income to get a pre-approval from the banks to help you take the first step.

Does anyone know a good source to find inventory?