Scott: if you search under "removable hardtop", I think you'll find a few discussions on this. JPS has two tops, one for about $2K, the other for $3.5K. They have different levels of attractiveness/ease of removal. IM also sell one and I think Chesil (in the UK) are working on a new one, having been dissatisfied with their old one. There may be others.
I don't have any experience of them myself, outside of research I've done on this site and with manufacturers, but Stan Galat, a senior member of this site, has expressed the view, as I recall, that it is unwise to purchase a hardtop other than at the time when you have the car built. He certainly appears to have had difficulties getting his anything close to weathertight.
Good luck
John.