Andrew,
Take a deep breath and realize that even the best educated guess can leave you with arrows that may not fly the best. Many factors will determine what arrow to shoot. There are a few questions that will help me and others identify arrows that will likely work. Please answer and we will see what recommendations come up.
1) What bow are you shooting? Longbow? Recurve? Recurves tend to be center shot while longbows typically are not. Longbows tend to require a weaker arrow that a recurve.
2)Brand of bow? Model of bow? Anybody reading this post that owns the same bow can tell you exactly what set-up they shoot for arrows. Helps take some of the guess work out of it.
3)Do you know if your bow has a fast flight string or dacron string? This can impact the spine required.
4) Have you read any books or other resources that discuss tuning a traditional bow? There are a few good resources that describe most of the factors that impact what arrow will fly best from your bow. A good starting point is the following link:
http://www.bowmaker.net/tuning.htm One thing to keep in the back of your mind. Form can have a significant impact on arrow spine to achieve good flight. After a few years of shooting traditional, I found a video on the internet on how to make your own version of a training tool called a form master. I made one and found out I was short drawing by about 1.5 inches. Once I learned a proper draw, my arrow length and spine requirement changed, meaning I needed to buy new arrows. What ever arrows you purchase, be careful about buying new arrows until you are confident in your form.
Learning traditional archery does not happen overnight. But that is part of what makes it so fun. There is always something to work on, new arrows or different bows. Don't be afraid to keep asking questions. Also spend some time on the shooting forum. There is a lot of good advice posted there.