I have seen a lot of inline volume controls made for 3.5mm jacks, but I'm wondering if there's such a thing built for a 1/4-inch jack. Does such a thing exist? Or will I have to get a 3.5mm version and then add adapters to both ends? (and if so, does adding such adapters carry any risk of reducing quality?)
There is no more risk of quality loss than is present in any adapter connection. The cables themselves are effectively the same as long as you use similar quality cable with equivalent shielding, it's just a difference in connection. What is the situation that you are trying to reduce the volume for? Generally 1/4 inch connections are used on more professional devices that should have their own volume or gain adjustments. There might also be impedance mismatch issues that could cause the inline volume control itself to not function ideally.