Paradise Pen Las Vegas
Paradise Pen is the only major pen store in Las Vegas. There are a couple of stores that sell a handful of pens, but just as a minor addition to their main stock of stationery and related items.
PP is located on the Strip, in the Fashion Show Mall, so it's easy to find, though you may have to deal with traffic and parking difficulties.
Their pen selection is large and diverse, from Lamy Safaris all the way up to Montblanc and beyond. Whether you're looking for something cheap, or blowing your retirement, they've probably got something in your price range. (The brands I checked off are purely from memory. There are probably others they carry as well.)
Ink selection isn't quite as diverse, but they have a good selection of at least the basic colors in a few brands.
I haven't ever paid attention to their paper selection, but I do know they carry a few things.
Service has always been excellent whenever I've gone in. The staff is always friendly, helpful, and knowledgeable.