The guys above are right about making your own strings-- saves a lot of time and money, and you can tweak string length to get it just right.
Flemish strings are very easy to make. You don't need a fancy jig. I use a one of the vertical 2x4s in the wall framing in my basement as my "jig". I've got 2 finishing nails 1" apart at the top (I build 2-bundle flemish strings), and several pairs of holes at varying distances from the top holes (I put finishing nails in the appropriate pair of bottom holes). I add 14" or 15" to the actual string length I want to end up with, and that's how far apart I space the pairs of nails. Tie a small loop at the end of each spool of B50 or whatever string material you're using, slip the loop over the top or bottom nail (depends on whether you'll have odd or even number of strands in each bundle), and wrap the string material up and down over the left-side top and bottom nails until you have desired # of strands. Repeat with the other color of string material around the right-side pair of nails. Wax the 2 bundles to hold each one together, slip the bundles off the bottom nails (leave them hanging from the top nails), then start twisting and overlaying as shown in one of the videos. Trickiest part is doing the lower loop, but it's easy once you've done one or 2. Or you can just do a bowyer's knot for the bottom loop.