During the past year, I mentioned to someone that I bought nomex thread at an online store and use it to sew patches onto my racing suit. I have forgotten who I promised to send the info, so here it is for everyone.
The store is The Thread Exchange (
www.thethreadexchange.com), and I got white nomex thread of size 207. The spool cost me less than $20 including shipping, and it will last me a long time. The website contains a ton of info on nomex thread so that you can make a decision on the type of thread that you want.
One advantage of nomex thread is its strength. Seems much more durable than cotton in surviving a cleaning process.