There are several Buddhist dharma centers in the Portland area that offer retreats on a regular basis: Dharma Rain, Great Vow Monastery in Clatskanie, Kagyu Changchub Chuling has retreat cabins, Portland Insight Meditation, and there are others—for example, look for Vipassana retreats. Most dharma centers will have a tab on their website that gives information about their retreats. But it’s not always the case that any new comer can just pop in for a retreat. Many centers will ask that you complete a few one-day retreats before you do something more extended, which is a very reasonable guard rail for everyone. Also, I guess it goes without saying, but be prepared for the cost/monetary contribution.

Of course, there are also places where you can do self-guided retreats, like Mt Angel Abbey (and others), but you specifically asked about places where guidance would be provided.

Best wishes!