Want to put in offer for Duplex, how to?

[Tyson Hosey]

I've only ever put in an offer to buy my primary residence, and for that I was pre-qualified for it then I went out looking for a house. Most of the lenders that I've talked to say "show us the house and we'll close in 30 days". Its completely backwards for me and I'm wondering how that usually works. I'm wanting to buy a "for sale by owner" duplex. Do I put in an offer then go to the lender? Do I go the lender with this potential property. How does this work? 

Side note: I'm purchasing it through my LLC, so if it's a small bank it would be a commercial real estate type loan. Individual/PML/lending companies probably wouldn't make much of a difference if it were an LLC or not.