Maybe you can smile and bow your head down a bit to show gratitude. In India, we don't say thank you so often. I just moved to USA to pursue my masters in University of Pennsylvania and it is only now that I notice how often people greet and say thank you. I guess this is how it is in the west. So maybe trying out what we do in India will help you a lot because it can be considered as a back up option when you're not feeling like saying thank you. And on days when you want to challenge yourself a bit, you can try to say thank you. If you're not shy enough, I would suggest you try to say thank you and stutter and then just stop without completing it and smile a bit and bow down. It will help in creating more awareness about stammering.