I have a new to me 16 ft jon boat with a 2017 Mercury 20HP 4 stroke.

I changed the lower unit oil yesterday and it was milky white/ assuming from water getting in.

I am going to replace the seals on the drain plugs, as they were not there and see if that fixes the issue.

1. What is a good place to find these seals? Can you find generic seals that will fit the plug, or is there a place in DFW that may have them in stock? I have found them online, but they take like 2 weeks to get in and charge $10 for shipping. The part is 12-895146.

2. If I am still getting water in the lower unit I will just take it to somebody. Any recommendations on where to take it? I am in Dallas at 75/635.

Try Mercury at Bass Pro Shops

Seals should fix it. Bass Pro or Cabelas will have them. If it doesn’t fix it, you might consider taking it to Brent over at Larry’s Marine in Plano.

I would get the proper size and type O Ring from a Mercury dealer, but you can get a good quality O ring from a plumbing supply for a lot less.
Tighten the drain plug screws to the recommended torque, Under tighten will leak, over tightened will easily strip.

Are you sure they are missing? A lot of times they stick to the housing.

Spot on. They were just stuck to the drain screw and in good shape. Unfortunately I think that means a larger issue. Do I need a lower unit seal kit? Is that DIY job? I'm pretty handy, but a newbie on outboards.

You could be optimistic and still blame it on the plug o-ring. I would replace it, torque the screw correctly, and run it a couple of trips, then check again. If you see that water has made it back into the gearcase, it's time for professional help. I'm a fairly decent DIY guy, but my buddy who is an outboard tech told me not to try that myself. There are a number of specialty tools involved, and quite a bit of know-how. If you end up getting a seal job done, do the water pump at the same time, of course.

Go watch this and decide if you have the skills.

https://www.youtube.com/watch?v=OGIHFmjd0Vo

From what my mechanic told me the crank and shift shift seals are the tough ones. The prop shift seals are not bad and most likely the culprits.

That's the one I suspect. Hook up a vacuum or pressure pump, take it to spec (10-15psi I believe), and you'll see if it's the prop seals or other seals.
Tim