18" alloys in a 8,8.5 are like you say easy to find, but finding a 8 x 18" alloy in a 5/108 with a 68 offset in bloody impossible, the minimum pdc you can go for is a 52 offset and they still stick out to far, iv bin to over 30 alloy wheel company for different wheels and thet are hard to find, yes k-tec sell them but they are a horrid wheel and are a 52 offset.
even antera made a one of set for ellite wheels in essex to fit mine with a 54 offset, put them on the car and they were about 2mm from the edge of the front arch, rear was ok but the front was terrible, even the shop thought so and sent them back to antera statting they were no good.
r-tek near hatfield have also tried many a wheel on mine to. got a lexani 20" wheel to fit the back with a 5/108 and a 50 offset with a 225/35/20" tyre on but clipped the brembos on the front, big shame as they looked huge and fitted a treat.
bottom line is alloys for the 225 are a bastard to find un less you are not fussed on the couple od designs out there.
the trophy whees are great and the new F1 allloys in black also look good