I have not found higher resolution graphic of FC crests that exist in the game, but there is a workaround if you (or someone you know) is willing to do the legwork for you. Honestly, I'd volunteer if I wasn't so tied up with my costume.

If you have a vector graphics program (like Adobe Illustrator or Inkscape), you can trace around the low res image and make a vector version that you can blow up to be as big as you want. Hopefully this will work for the purpose of embroidery designs. Illustrator costs money, but you can also accomplish the same thing with Inkscape, which is free. Lots of cosplayers use Inkscape for blueprints, so you can probably find a simple tutorial explaining how to do what I am talking about.

You could also turn on the FC crest on a piece of armor where it appears to be very large (the CUL apron comes to mind -- just an example), but that will still be very blurry for you.