I have an sks with a trimount, what is a good scope to use?

Pretty much any fairly priced 4x or 6x hunting scope from any reputable scope maker will be more accurate than that rifle so look for the biggest objectives for light gathering, and the widest field of views at 100 yards. If you can get some sort of mil-dot reticle, that would be a plus as well. Look to spend around what you probably paid for the gun, maybe a litle more.

If your trimount is one of the dust-cover replacement scope mounts, there is NO good scope to use on it. The dust cover shifts with each shot, and you're better off using iron sights than a dust cover scope mount.

That having been said, look into the "scout" mount that replaces your rear sight leaf. Using that mount, you can restore your rifle to its original configuration readily and when properly adjusted, you will have a very solid scope mount.

As for scope, avoid NCSTAR and most of the cheaper scopes. You should expect to spend around $100 for a decent scope with any variable power. In cheaper scopes, you can lucky with non-adjustable scopes, but any scope that zooms and costs under $100 is likely to be a piece of junk. It took me a number of scopes before I finally learned my lesson on that point.