I just bought a Marin Muirwoods 29er, but it's not "really" a 29er. It's got 29x1.6 tires. The tires (Continental TownRides) say 622-42 on the sidewall. Therein lies the problem.

I've found "29er" tubes, but they generally advertise a 29x1.75 minimum. But I can't find a 700c tube that advertises support for more than 700x38. Then, if this isn't fun enough, the rims are drilled for Schraeder valves. I know I can get an adapter sleeve, but I'd prefer to keep it "native".

Anybody have any idea where I can get a tube for these tires? Will 700x38 or 29x1.75 tubes work?

Can't help you on the schraeder valves, but I run Conti Tour 700 all tubes with a presta valve. They're marked "28/47 - 609/642" and "27/28 x 1 1/8 - 1.75 x 2 . 700 x 28/47B/C" which I interpret to meet your other requirements. They came from www.mec.ca

Now that I look on the MEC site, I see:
https://www.mec.ca/Products/product_d...=1210881030439
that might fit the bill.

My spare is a presta valved WTB. The box says "MTB Tube" for 700x52/47c. Just run a schrader to presta adaptor.

Schwalbe has 622-42 (700c X 40) tubes, check out AV17 & AV19:

https://www.schwalbetires.com/bike_ti...chraeder_valve

I typically run tubes a little smaller than the tire size, so 700x38 would be fine by me.

Ditto. Running 23c tubes on my 32c tires for months now with no problems.