As Joe says - 195/50s fit the standard front rims (I have A048s on mine).
As for yoko being the only supplier for that size it is that Lotus worked with yokohama to develop tyres for the Elise (earlier models used bridgestone but that tyre is no longer available). No other manufacturer makes a tyre in that size because no other car uses it (combination of light weight but high performance requirements etc) and once the originals wear out most owners go up to the 195/50 so the market is very small.
PS: moved to correct forum too
