spark plugs

I have 99 grand prix gtp, what is the best spark plug to use? It is bone stock and the highest octane fuel I can get around here is 89 with 10% ethanol.

Bone stock, you can run a lot of things. Factory plugs in Autolites are 606s. You can drop down one heat range to 605s if you want.

Is the 89 octane with 10% ethanol sufficient for a gtp?

If your at high altitudes yes. As altitude increases octane ratings go down.

I am in the middle of Kansas. The only fuel available is 87 or 89 with ethanol.

89 octane isn't great, but as long as you are on stock pulley and PCM it should be safe. The car will just pull timing as it reads knock.