First off welcome and ask as many question as you like and enjoy the site.
Shorts I would highly recommend Toezup, great fight shorts and all around great clothing for a great price. They will have rash guards soon I believe as well. As far as equipment I would look at muay thai factory. Here are the websites for both.
I am new to mmaforum but I am on some other sites. If your interested in a custom mouth guard please visit my site at www.customguards.net If you ever have any questions please let me know, all of my information is on there. Another good site to visit for gear is www.mmauniverse.com I have gotten a large amount of supplies from there.