On an earlier thread, Metler Metrosene Plus seemed to work out well for the poster.
It is a polyester covered polyester like many of the others currently on the market, but many find it superior for outdoor use to Coats, Gutermann's and the others available at the chains. It works well with modern machines, is strong enough for tents, and is quite durable. Local sewing shops are most likely to have it.
Sew with low enough tension so that the fabric will not pucker (experiment with swatches first), especially if using silnylon. When adding webbings and tapes in the sewing, you will need to tighten the tension adjustments for both threads. Again, more experimentation with swatches to get it right.
All the brands have heavier threads that are almost unbreakable, and are appealing for this reason, but often create problems in the machines, especially the more modern ones.
Here are some pix of a silnylon fly sewn for a Hubba with MM plus: