They are difficult to find wheels to suit as the offset is larger than most other cars. IS250/350 wheels fit, the centre bore, offset and wheel nuts are all the same, they can also be wider in the 17, 18 and 19 inch rims. They fit straight in, but the front end won't take anything wider than a 225, the rear tyre width limit is 245 before something rubs. Its the cheapest and most practical wheel upgrade you are going to find, I use 17 x 8's and use 225's on the front and 245's on the back which is as wide as you can go on a 8 inch rim legally. Pictures show how flush they are to the guards with a piece of stainless pipe (front and rear), wheels tuck in as the suspension is compressed. Can't go wider by using more offset, as the wheels start touching the suspension on the inside, less offset and they stick out more and hit the guards, which in turn means fitting narrower tyres or flaring the guards.
Only four of the many Lexus styles shown on an early IS200/300, black rims are 16 inch rims from an IS250 which were v cheap and already fitted with Potenza tyres and had been rattle canned black by the previous owner, silver paint underneath. 16's are OK as the higher sidewall gives a smoother ride with harder suspensions. If you fit aftermarket wheels and the centre bore is different and no adaptor is fitted, then the shock of going over a big pothole is taken solely by the studs and if they bend then you will have a permanent vibration. Enkei's and others are lighter but will cost a lot more plus the new tyres you'll need as well, if buying new.