There are a couple of other companies, and independant sellers who produce some top grade silicon molds. Below you can find links to their websites, and images of the molds that I own.
A&K Studios is a small independant company setup by two people from the Hirst Arts forum, in order to make quality silicon molds, that compliment the Hirst Arts range. The molds are top quality and reasonably priced. I currently own the gravestone and Moria molds, pictured below.
Woodland Scenics is a professional company, mainly serving the model railway scene. They also provide other modelling tools and accessories. I find their molds are slightly harder to cast and demould than the other molds that I have. I currently own 3 of their molds. C1233, C1236, and C1243. You can see an image of each of the molds below.
Linka are another professional company, mainly aimed at the railway modelling market, and this is reflected in the scale of their molds. Even though I myself am relatively new to 3d terrain generation, I've actually had my Linka molds since I was a child (some 25 odd years), and they still work. Although they were never used when I was a child, they were bought for use with my model railway. The molds I have were part of a set, and therefore contain many of their molds. I don't recall which molds were included, and therefore am not including an image of them below.
Klingbeil is another person from the Hirst Arts forum, who has created the Girder mold. At the current time, Klingbeil doesn't have a functioning website. The girder mold is a little bigger than the Hirst Arts mold, but made of a similar material. It is well made, and includes then name of the mold on it. My intial casts look fine, although I currently have yet to construct anything with them. I believe Klingbeil is looking to expand the range of molds that he has available in the future. Below is an image of the girder mold.