I'm looking to find an adjustable mandrel? Mainly looking for 5/8 and 1/2. It seems the only place that sells them are CSUSA. Does anybody else know a place that sells them?
I am guessing you are looking for game call mandrels. Try this guy thirdwheelrider | eBay
He has the sizes you asked about in either straight shanks or MT shanks. If you want shorter versions, he also makes them.
I just use my 3-jaw scrolling chuck and a length of steel rod of the appropriate size makes a great mandrel ... very cheap and easy to replace if something happens to it.
In many cases, you don't even need to buy the rod stock. A single carriage bolt of the appropriate size and length would work!