The Luna and Celestia are probably cheapest gotten through the princess twin packs. I believe they're still 2 for £15 at Argos, and then you can sell or trade Pinkie and Rarity :> They're fairly common in most supermarkets and all now so it's very likely they'll go on sale in more places.
For Nightmare Moon...TRU US do ship to the UK for about $7 and let you pay the import VAT in advance, but if you want her loose then you're likely better off buying one of the bootlegs off one of the Chinese/HK Ebay sellers. I've heard they're decent quality but I've yet to buy anything from them so I cannot personally vouch for that.