Standard Bullitt ones, should be able to get them from any Larry vs Harry supplier. Don't really know where you can get them, but I have found an EU source for me.
For more consultation you can have a look into the assembly manual
I think you should be able to produce similar results with regular bolts, washers and nyloc nuts. You would need some kind of standoff or a larger L bracket, but I guess it would still be cheaper than paying LvH tax.
Ideal scenario would be to build your base a bit wider so it sits on top of the Bullitt frame, so you could just use regular L-brackets for the side panels. Would recommend using a fairly thick wooden base and countersunk hex bolts to attach it to the bike.