I've looked around and there are a few places online where you can design your own brands.
There are places like L & H Branding Irons. They seem to have really good quality. L & H Branding Irons - We've got your brand.
There is also Valley Vet. Freeze Brand Custom 3" L H MFG CO (Identification - Brands - Freeze)
I linked to the 3" iron because that's as small as I would want to go. They do have 2" brands, but the bigger ones are more easily seen from longer distances.
Pretty much anywhere you go, a custom design is going to be more expensive. If you just wanted to do your initials and run them together somehow, you could just buy the individual letters from L&H for about $85 and have the brander put them on how you wanted. Most custom jobs are going to be $100+ just for the iron.
As for design, you can get pretty much anything you want on there, it's just a matter of cost. A lot of people use their initials run together. That's what we had for our cattle when I was a kid. My Grandpa's initials were N.R. And that's the brand that we used. I had considered getting my brand made the same way but with J.R. Instead, but I've since changed my mind and decided that I want something more original. Something that it's unlikely anyone else will have...anywhere.
I've attached a drawing (not sure how it'll turn out) kind of showing how those initial brands look.
In addition to the brand, though, I would also take added security measures like cameras and locks. If you happen to see them again, take down the license plate number from their vehicle. Make sure you know where they are living if they live close. Put up No Trespassing signs on every single entrance to your property and along your fencelines.
Here is a nice video about the process of the whole thing done by the same folks.