Ruger will not sell or fit a different cylinder than what came with the gun originally.
Period.
They can tell by the serial number what the original configuration was. That said, if you find one on ebay, gun broker, or someone here on the forum, be sure you know what you need. Do you have an old model Vaquero with the larger frame or the new model with the mod-size frame - the cylinders will not interchange. Measure your existing cylinder and look for one that size or slightly larger (lengthwise, from the front bearing surface to the back side of the star.) You may be able to shim if you have a shorter one, but that may open up the barrel/cylinder gap. Search for info here on the forum, I have written extensive commentary on just this subject describing my own experiences.