I do all my shopping at
www.honigs.com. Others will recommend other places, and they are good too. I just prefer Honigs.
Get the V-neck shirt. I don't like the look of the mesh, but that's mainly preference. You may want to ask around to see what other officials in your area wear. As for pants, the pants on honigs are all good - pleated, non-belted seem to be the favorite of most these days. They are certainly my preference.
Whistle - I use the Fox-40 CMG (has the rubber around the tip, as I'm a "chomper"), but that or the Fox-40 is the way to go. Some use the Fox-40 mini, but I don't like the sound as well.