If the condo isn't listed, you'll need to find and contact the owner.
Your first stop for information of any property-- whether for sale or not-- is http://www.portlandmaps.com/
Just enter the address of the property, then click on the "assessor" on the banner menu. That should give you the owner's name. For a condo building, there will be a list of the units and, if you have the unit number, just find it and click for owner information.
If you don't know the address, enter the street intersection and click on the map at the point where you think the building is located.
With owner name and address, you have a way to make snail mail contact.
If you suspect the unit is for sale by owner, you'll probably find it on the Portland Craig's List: http://portland.craigslist.org/
, or one of the many for sale by owner websites.
Or, you can have a real estate agent do the legwork for you, then negotiate with the owner on your behalf, if the owner wishes to sell.
Hope this is helpful. Best of luck in your search!
Ed Burnham, Real Estate Broker
Windermere/Cronin & Caplin Realty Group, Inc
825 NE Multnomah Street, Suite 120
Portland, OR 97232
Visit my website: http://www.edburnhamhomes.com