I am in Germany but I could buy HirstArts molds from a german RPG shop. While the link list on Bruce Hirst's Site is less than comprehensive a short websearch brought up these two shops in the UK. (I put them in Spoilers because I am not quite sure how to handle this. I do not know these shops neither am I advertising using them!)
You ask for tips and pitfalls:
Due to RL constraints I did not cast very much till now. I got the Roman Watchtower, SciFiBuilding and Scematic Pyramid Mold which I could cast with regular plaster. I got two machinery molds which did not cast so well with the regular plaster, causing cracks, too much bubbles etc. I am yet to buy & try some dental plaster or resin with them.
: I tried this with Acrylic Paints. The Problem is: The dry plaster kind of sucks the paint right out of the brush while you're basing the building. For the next projects I will try a spray-on basing beforehand.
I also experimanted with mixing acrylic paint into the water before adding plaster. The color will change (due to the white/grey plaster) but it works pretty well!
If you like I could upload some pictures (But it is basically one tower + some experiments atm). Also feel free to ask me any questions!