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I've got 38t coming with mine when my mason bokeh arrives in June. Not sure if I should have gone for 40t though.
French_Touch-
Depends how much you'll be riding it on road and on the cassette, I guess. 38-9 is still 4.2, which is a bit harder than 50-12, so if you were riding a road compact you've (not quite) lost your top gear, but not much else. If you don't mind freewheeling at >50kph and weren't planning on a very fast group road ride, I can't see there being an issue.
I'm running 2 wheelsets and will have the 40T upfront (largely because that's what I could get). Am putting a 9-36 cassette on the road wheels and 10-44 on the gravel ones, as - according to Malcolm - Ekar will allow me to do that without changing chain length. I'm hoping that will cover all the bases.
RecoveryRide
I got mine from bike24. Couldn't find a 38t chainring though, I will also have a 40t with 10-44.