Radiators last a long time so most people do not buy them very often and may not have much idea where to start.
I would start by looking at the huge collection available at some of the online retailers just to get an idea of what is available.
You may want something unusul by a unique piece of artwork may be beyond your price range and arty rads are not always practical. I would recommend looking at the range produced by the manufacturer Stelrad.
They are one of the major suppliers of radiators in the UK but their modern and designer ranges are really worth considering.