I have great credit but I am in a position to buy it outright with cash.

Are you credit worthy(have you seen your report recently)? 

If you try to get a seller financed second mortgage you may have to call around to a bunch of local banks and credit unions.

@William MurdenIf there is a broker involved they might have looked at the supply index but I've rarely seen brokers do that and even then I'd do my own due diligence on it.I recommend picking up a copy of the Self Storage Almanac from Mini Storage Messenger.https://www.ministoragemessenger.com/product/2018-self-storage-almanac/As far as doing your own research on the supply index, @Mark Byrge is correct.

Is there a way on loans to put some sort of contingency on the loan like, if I default that he will be liable without having the loan show up on his credit? 

A key difference with section 506 investments concerns the products themselves: they are not regulated by the SEC and Investment Company Act of 1940 which governs stocks and mutual funds.  

From what I've seen, traditional financing through a small bank/credit union is probably my best bet.

You should have started th eeviction April 2nd.Unless your state is different than most a security deposit can not be used to cover rent.Serve the evictions on all three tenants and most likely they will pay up to protect their credit.

We both have good credit and established ties with a commercial lender who will do a non recourse loan for the other 80%. 

Not only that I rent to people that most people here would disqualify on grounds of credit history.