You will probaly have to special order through a lumber yard or home center. That doesn't mean it is custom; just not stocked in the store.
One of my customers special ordered one through Home Depot. It was an outside door - as most of them are - but they got a good one.
An interior door is usually 1-3/8" thick. Be sure to specifi that - if you can - so it will fit the interior door jamb. An exterior door is 1-3/4" thick.