at most fabric/sewing/art-crafts shops. It's a mono-filament nylon, damned near invisible and a whole spool might set you back about $1.00.....that's a lifetime supply !
Comes time to finish the stitch row, either get your wife's advice, or simply tie it off by sewing thru a knot, then a TINY touch of crazy glue so it doesn't try to loosen over time.
The stuff is almost completely clear...or MAY have a bit of tint to it....not enough to stand out.

OK, Ulf and I have done our share, now it's up to you..........I wish you well with this...........
Keep the board posted on progress............OK ?