Clark W. Griswold
Join Date: Mar 2014
Location: ,location, location
Posts: 14,624
Bikes: Foundry Chilkoot Ti W/Ultegra Di2, Salsa Timberjack Ti, Cinelli Mash Work RandoCross Fun Time Machine, 1x9 XT Parts Hybrid, Co-Motion Cascadia, Specialized Langster, Phil Wood Apple VeloXS Frame (w/DA 7400), R+M Supercharger2 Rohloff, Habanero Ti 26
This is the mountain bike subforum just FYI. In terms of finding a freewheel or cassette bring your bike down to your local bike shop and they can help you find one. It is unlikely you are going to find a 38t freewheel or cassette probably the highest you will find new is going to be 34. You might find some old stuff assuming you need a freewheel which is different from a cassette. Some of the older stuff could be custom built and you might find something that big on a custom built freewheel.
Also consider your derailleur might not be able to handle something that large.