Old mtb wheels is a good shout, that’s what I did for my 650b wheels, bad ebay listing for some DT Swiss XR1501, 25mm internal and I picked them up for a ridiculously low price, £20 for some 12mm end caps for the front wheel and they were the perfect pair of wheels for gravel riding. Mtb rims tend to be lighter than the equivalent width gravel ones. They’re not rated for the higher pressures of road wheels but as I run my 40mm tyres at 34/37psi and the 47s at 31/33 I’m not too worried! The road tyres at 55psi though I’m definitely sticking with a proper road rim.