What size are 200/580/15 tyres and what width rim will i need, thanks.

200mm wide
580mm diameter
15" rim

The nearest road tyre size to those is the 195 50 15, they will be ok on either a 6", 6.5" or 7" rim.

200/580/15 is 200mm wide x 580mm diameter x 15"
a 195/50/15 is 195mm wide x 576mm diameter x15"

basically 195/50/15 ive got same size fitted to 6 1/2" wide rims which i think is about the minimum without distorting them but think they can be fitted to around 6-7" rims

have you found somewhere cheap that sells 15" slicks. There are not that mamy places.