Absolutely! You can get the Santa letterhead templates from a lot of websites and most of them are free. You do not need to sign up for a membership or pay the delivery fee or something like that. You can get the template usually on the parenting forum online because truthfully, the one who gives you a present and answers the Santa letter is the parents. You will get a lot of new experiences from these templates to reply to Santa's letter with his signature!
Since the one who gives the present is adults, then, they will also use the templates to reply letter to Santa. Mostly, they are parents. Although some people oppose the idea of lying to the children about Santa Claus, some parents believe that they will let the kids believe to improve their imaginative ability. But for some kids who lost their parents, there is a community where someone wears a costume as Santa. That person will use the template to reply to the letter to the kids. And so, the templates are used by adults because the figure of Santa is also an adult or elder man.
On the design of the templates alone, you usually write "from the desk of Santa" on the top of the paper and "sincerely" or "love always" on the right bottom of the template. But for the overall letter, you should reply to the wish and thank-you letter that the kids sent earlier. If it is a wish, then, you write an appreciation of their deeds throughout the year and encourage them to do more good things in the next year. For a thank-you letter, you can write a thank-you letter back.
The easiest source to get the inspiration for Santa's letterhead template is the internet. There are a lot of articles and online forums that share the templates of the letterhead with Santa's sign. In the articles, you can see how Santa is dressed up all around the world. If you look upon online forums, you can read how the contributors make the content and design of the templates. It is also possible to get the idea from movies or TV shows about Santa Claus that you can watch around Christmas time in December.