Just to clarify, a "portfolio loan" has nothing to do with having a portfolio (or even having other properties); it just means that the bank holds the loan in-house (using it's own funds) as opposed to selling it on the secondary market. And because the bank holds the loan in-house, they make the lending rules, not the secondary mortgage buyers like Fannie Mae and HUD.
And when they make the rules, they can do things like lending against the deal (instead of just against your financials), lending when they want (for example, refi immediately after purchase, which most banks won't do), and lending any amount they want (they can lend at higher LTVs if they want to).
That's why portfolio loans are great -- they can be negotiated however you and the lending institution agree (though they generally have their own guidelines for most borrowers).
Now, how do you find these lending institutions?
Normally, portfolio lenders are very small, local banks (think 1-4 branches in one or two adjacent towns/cities) and you essentially build the relationship on a case-by-case basis. Start by talking to the VP of Commercial Lending and verify that they offer portfolio loans. If so, verify that they'll against against residential property, as many will only make "commercial" loans. And lastly, find out what their specific terms generally are (again, it may be negotiable, but it's where they will start).
Generally -- in my experience, at least -- portfolio loans will be interest-only, amortized over 15-20 years with a 1-5 year balloon. Interest rates are around 6-8%, with 1-3 points upfront.
They will look at your experience, the specific deal/property, your financials, your credit and a host of other factors. They may ask that you open a personal or business account at their bank or perhaps collateralize the loan(s) with something other than the property(s). For example, the first bank I worked with offered to finance up to $250K at a time, but I needed to keep a $50K CD at their bank for the first year, which was held as collateral. This was mostly because I had little experience at the time.
The other nice thing about portfolio loans is that they can be much faster to fund than conventional loans (less than 2 weeks), have in-house appraisers in many cases, and will actually know who you are, so you get the benefits of a good relationship.