Its really going to depend on the agent. I do some rent to own and the biggest problem I've run into is that other agents rarely understand lease purchase/lease option deals and are therefore resistant. However if you can explain how it is beneficial to all parties you'll have some luck specifically with properties with high DOM's and those that are priced a little too high so they are not moving. Commissions will generally run from 1-2 months rent up front with remainder of commision at closing, or perhaps a few % up front with remainder at closing. Ideally of all commissions get paid up front you'll get less resistance from agents/sellers. But as in all real estate deals....everything is negotiable.
However if you're thinking about a sandwich L/O, with an agent involved - good luck. Count on TONS of resistance.