Ive got a tool called Market Snapshot. I can input the zip code and it will give me several charts, one of them being DOM. Email me the zip code and Id be more than happy to send it over to you. Or Joshua's answer is good too if you have FMLS.
You can do it several different ways, depending on what you are interested in.
Trulia has them.
If you are a member of the FMLS, you can actually download an absorption report, complete with DOM.
You can also do a search in FMLS and then click "STATISTICS" to see average DOM.
Hope that helps