If you must, try a spool of UTC. Set you back 2 bucks or less. It's a flat thread so you can do a few things with it that you can not do with Uni, like split it and insert dubbing, then twist. Get a heavy version, such as 140, so you can also spin deer hair and other heavy applications if you find your 6/0 Uni is not up to the task.
I would have suggested Monocord, but I don't think it's being made anymore.
In general, the 8/0 UNI is the way to go for most trout flies in my experience. Stick with it unless there is a specific application that requires something else.
Of course, you may want to go the opposite route and get very small, such as the Gordon Griffiths 14/0. That may sound fragile, but it really isn't at all.
So, one spool of UTC 140, and one spool of Gordon Griffiths 14/0.
I am curious about Roman Moser Power Silk, but have never used it. I have tried Kevlar, but lack sufficient experience to say one way or the other. It is very strong and will cut through some materials. It even hesitates a bit when being cut with scissors!
Then go to the craft store and marvel at the metallic threads and wonder about those possibilities, such as tying with it, leaving a long tag, and ribbing with that, even on your dry flies, for that little bit of something different.
Soon, you will have to buy one of those spool racks for the several dozen spools of thread you own, most of which you never use because you always have 8/0 Uni in black, olive, grey, brown or light cahill in your bobbin.
;-)
