Your best bet is to be 100% honest and open from the start, write a letter explaining why your credit isn't where it should be. I have had clients do this many times and have yet to not find them somewhere to live. You will have better luck with an individual owner/landlord than with a managed listing.
The agent you use may not be your problem but like others have said the agent representing the landlord will look out for their clients best interest and not advise the landlord to not do the credit and background checks.
Not all landlords will eliminate you based on blemishes on background and credit. It sometimes depends on the length of time since it occured. If you have verifyable income more than 3 1/2 times the rental amount and have been employed in the same field for several years, they may be more forgiving. Even if your credit is not perfect, but your rental history is good, they may choose to ignore some of the credit and background problems. It will eliminate you with some landlords but not all. The longer it has been since there was a problem, the better chance you will have. Do be honest about your past when they ask because it's very easy to check now.
There are some landlords that don't check and don't care. Personally, I am not one of them and wouldn't suggest my client to rent from one.
The best thing to do will be find a REALTOR you can trust, explain your situation, and they will work with the other agent, with your best interests in mind. You just need to be honest and upfront on your application. Honesty goes along way!
You are not likely to find any landlord that will not use a background and credit check on their tenants.
If you know you have a blemish on your background information the best thing you can do is write a letter to your future landlord and explain the circumstances of the issue and how it has been resolved. A simple and sincere explanation is all that is needed. A reasonable landlord will appreciate that you are upfront and hopefully will appreciate that you've solved your problems and are ready to move forward with the intention of not making the same mistakes again.
Honesty is the best policy.
I am a Realtor and I work in property manager. It is the landlord that makes the background and credit check request. I don't know a Realtor that would list a rental property on MLS and advise the landlord not to check for credit and rental history of an applicant. It's just not good practice.
It is unfortunate that perfectly good folks may have nad their credit effected at no fault of their own (for example following a divorce or illness). My suggestion would be to try Craig's List, the Chronicle or other newspapers. You can find a landlord that can't stand the sight of real estate agents! lol