What professional installs indoor direct vent tankless water heaters in the USA?

I had mine installed by furnace installer

Any retailer of tankless water heaters should certainly be able to recommend some local contractors in your area. As far as cost... They are not cheap. Depending upon what you actually buy, the installation can be expensive because you may have to "vent" the unit to the outside & you may need to install a 220 volt line & plug for it. Find out what kind of a unit that you actually need first, so you know what has to be done to install it. Then, I would get a couple of estimates. I have looked into those myself and personally, I decided they're not worth the trouble. Good luck!

keep calling plumbers. Most haven't installed them into houses. Most are against having them in houses because they don't work with them often. I've talked with 5 plumbers, 3 of them did their best to get me to return the tankless water heater and buy a cheap tank based. I found two plumbers that have installed them into a house and don't see a problem with it.

That would be the plumbers and the price will vary depending on the area you live in. Call any of the local companies and I'm sure they would like to give you a bid (which should be free) call at least three.