All over but generally up the north as they are way cheaper than down here. All the places I rang in the UK delivered to Ireland so no worries there either. Haldane fisher in Newry is a great place to go if you want to find specifics etc of what you require, as they have a whole sales team, you can ring them. They have all the aga accessories anyway which of course are dear but of course lovely. Twyford cookers in UK, Classic cookers in Down.
Or here there is the Aga shop Dun Laoghaire, H&F enterprises in Tipperary, they do reconditioned Aga's at not bad prices and are very reputable. Usually at the home exhibitions in the RDS. Just get your spec and email all these places adn they give a price but don't forget to ask for the extras to be included if you need them, e.g. flue, plinth,installation etc.
If you want an electric Aga you should get a new one as they are only new out and word on the grapevine is that reconditioned ones would only give you problems. Hard to find also. Reconditioned oil is €4-5,000 whereas a new oil is €8,000ish I think, electric €10,500ish. They are just ballpark figures.