I stuck to 17s same size. Cost effective tires are Toyo T1 Sports, Champiron or Kumho.
Am using the Toyo 17s rear now and find them very grippy. Cost less than RM800 per pc. Fronts are less than RM500 per pc.
The Champirons and Kumhos though no personal experience, some rempit kings are using them with pretty good experience thus far. And they are cheap as heck. If these guys can use them, then they're pretty solid I would say.
If you are upgrading rims to 18s for cosmetics, probably give the above a try, or Goodyear Asym. Some are using them and are very happy as well.
I bought a spare kit for long drives, just in case. For in Klang Valley use, i don't carry the spare around normally. U can get it from Euro Tuner or Lim Tayar or Quick Save. Price varies.