A good option for collecting enough hair for a sample that won't leave "holes" in the hair would be to use a pair of thinning/blending shears in several different areas of your growing hair.

These only cut a few hairs out each time you cut into the hair. Pull the hair up in small sections and cut as close to scalp as you can to get the longest length possible. Try to cut only once in each area, collect those hairs, then move on randomly to other areas until you have enough hair for a sample to send in.

You can buy them at Sally's Beauty or order a pair online:
http://www.amazon.com/SE-6-5-Hair-Thinni...B000V6WDUS