There are plenty of properties in the Clackamas area listed in Portland's multiple listing service (~130 single family homes under $400,000).
Most that I saw were asking for conventionally financed mortgages or cash offers. Most homeowners want to take the equity out of their homes to put toward their downpayment on the next home. If you find a rent to own or lease option, be aware you will likely need to put down a non-refundable deposit and negotiate how much of your payment will go toward your eventual purchase option.
If you're looking for rent to own or lease options, you may want to watch and/or post to Craigslist. Many investors use it to market their properties.
You can also enlist the services of a licensed real estate agent (warning, I am one!) who can advise you on financing, neighborhoods, available properties with lease option terms, and the rest of the 'process'.... more