see the above post for the tire sizes. You'll have to hunt around a bit as most places that have used tires won't necessarily have the size you need- 18" are difficult to find and I imagine 19" would be even rarer.. but definitely search craigslist and if all else fails, I did purchase a used tire on ebay with fine results...

I saw in another thread you were looking for wheels... you may be able to find a nice set of used wheels and tires, but, as stated, be careful in that there are a lot more options that don't fit on our cars than do... if you're looking on craigslist, I would stick with wheels that fit/came off of a Mercedes