What tool can I use to cut wall studs straight?
A handsaw, a square, and practice. While it seems common for folks starting out to reject this option, (practice seems to be out of style) it works. But you need to accept the idea that you'll actually have to make some practice cuts and get in the habit of noticing how the saw is cutting and learn to keep it on track.

A mitre box. You can make one from wood of whatever size you need.