Originally Posted by Matt Evans

The worlds your (or your mates) oyster.

195/50/15 tyres are a very popular size, and there are hundreds of tyres available in that size.

Something close to original would either be Dunlop SP2000 (if you can still get em) or Pirelli p6000. The dunlops have a plain (ish) sidewall and a similar uniform tread pattern to the D40's (from memory)

I'd be inclined to buy 4 matching quality tyres and enjoy the car.