I'd email Adam and ask him to send you one. Replacement 5-string nuts aren't easy to find. I had a look recently for a bone 5-string nut for a bass of mine and ended up buying a blank to make one from.

You could measure the width of the neck at the nut and search the web for a suitable 5 string replacement nut. I'd go for bone or a bone substitute, not just cheap plastic (which is what should have been on there). Not a Fender style one that's got square edges all round, but one with a rounded edge to it. Graphtec do some replacement Tusq nuts for 5 string basses, but you do need to get one of a suitable width and height and depth. If it's a bit too deep or too high you can sand it down, but you certainly need it the right width, or no more than 1mm over, otherwise the nut slots will be in the wrong place. https://graphtech.com/collections/tusq-nuts-bass The PQ-1412-00 is the shape you need but I have no idea if its dimensions are correct.

But you may have to get a blank and cut the slots and do some final shaping yourself. But it does help to have the original to copy from. You just stick it in place with some PVA or Titebond wood glue. If you fit the stings loosely, slip the nut under that after gluing the edges that will touch the wood, and then tighten the strings up a bit, then the string pressure will hold it in place. Leave it to dry for 24 hours before doing anything else.

You can cut a bass nut just using needle files. I generally use a round one and just use the tip to get an appropriate diameter slot. You can use a small thin bladed saw to help start cutting the nut slots to get the slot centres in the right place.