Matt, I just found a few power supplies on ebay which may suit your needs very well... Im actually in conversation with one of them to find out about the suitability for semi-outdoor use.. if its all good I'll be buying it.. 13.8v 25A, $130. Australian made.
The problem with most of these power supplies, is that they are not suitable for outdoor use where they may be exposed to dew and moisture... If need be, I'd build a ventilated box for it..
there are more options for you obviously, being that it will be inside an observatory,.. just needs some basic cover and it would be fine.