I have one bike with trigger shifters and one with a twist shifter. I don't care for the twist shifter and will switch it out when I can. But, as fietbob said/implied, your options depend on your current set-up, you which you provide no info. Issues that will determine your options: bar diameter -- for a trekking bar, it's probably mountain bike standard. What is your current gearing front/rear? Who makes your derailleurs? Are you interested in bar-end shifters? Then you need to know the internal dimensions of your handlebars.
I suspect you have several options open to you, but I don't think anyone can suggest a specific product without some details of your current situation. I've always used twist shifters on my trekking bars because it's what was available for my rear hub, but I'm looking into changing my rear hub just to get rid of the twist shifter.