I know you would really like to stick with your size 8 needles, but sometimes that's not always possible. Gauge is different for every person, and you could be buying yarn from now till Christmas and still not find the right yarn to get the gauge you need for your pattern.
I have two recommendations for you: First, if possible, contact the person/company who created the pattern and ask them what kind of yarn they used.
My second recommendation is to get a couple of different sized needles. You don't have to get extremely expensive ones, but do get a couple of sizes on either side of 8 (6, 7, 9, and 10, for example). Then, try your gauge again with these various needle sizes. After all, if you knit for any length of time, you will use these needles, so they definitely will not be a waste of money!