Hey, check out this other thread that basically covers exactly what you are asking. The first post has a lot of good information, including a recommendation for cheaper tires being sized 225/45R18.
what-wheels-will-fit-my-altima-will-it- ... 46628.html
You can use a tire size calculator on a site like Tire rack dot com and that will help you find tires that match your stock diameter as best as possible. Narrower tires are cheaper and smaller rimmed tires are cheaper.
I don't recommend spacers as much as I recommend wheels with the proper offset. You can measure the gap from the wheel to the fender and see if you have room for some spacers though and buy that thickness. If you have money for spacers though, you should be getting better tires instead (in my opinion). Tires are a very important part for your car. Some argue it's the most important upgrade.