How can I find a list of homes that are not currently for sale but that may have an ADU, so that I can start to reach out to these owners about any potential interest in selling?

I would make you a list of sites for quick use. 

I am a first time real estate investor looking the following property : https://www.redfin.com/OR/Forest-Grove/2418-Cedar-Cir-97116/...its a pretty large duplex in Forest Grove OR,  the trick is though, its on a lot zoned for a 4 plex so I am lookinginto house hacking, converting into a triplex, then living in one unit while renting out the remaining two unitsThe left garage has been converted to living space, with a living room, half bath, a laundry room with a sink and some counters/cupboards and a bedroom, it is a part of what was an existing 3bd unitIf I were to acquire the asset, I would first attempt to convert that space into its own unit The line items as I see (be in mind, I am totally new to this) would be to >put up dry wall where the door to the rest of the house is,>add a shower to the half bathroom >install a water heater in the adjacent 3bd unit> install a kitchen in the laundry room, (stove , oven , sink, counter space)>add a shower to the half bath Assuming I can convert it to a 3plex, I could at-least cash flow on my investment after I leave My plan would be to put minimum down payment of 3.5% (FHA loan) that way I can have a decent amount of capital free to do the renovationsThe only concern is when I crunch the numbers to get my ROI, I have no idea how much its going to cost to convert the additional living space to a third unitAny experienced investors here know how I can estimate this before I try to make an offer?

@Christie Dunivant Come to our women’s group tonight or March 25th listed on ft lauderdale BP events page.

I listed the property, tenant agreed to rent, I submitted the RFTA to housing with the rent amount, waiting for a week,  they emailed me and negotiated the rent, sent me the affordable worksheet showing the accepted rent, housing payment and tenant payment.

I’m on about every single wholesaler’s direct list in the area.

So we're not able to list on the MLS as they are not affixed to the ground.

VA cannot list the home until they complete the foreclosure and take possession.

Also, local Facebook groups and Zillow can be other good places to find MTR listings as well. 

If you are spending a lot of personal time there, I would speak with a CPA about whether or not to continue to list it as a depreciable investment asset on your tax returns.