Where to begin? I was w/ you till the last comment Jay. First of all I really don't want to come off as a condescending know-it-all like many of the guys on these forums, BUT not wanting to experiment w/ mixing colors yourself indicates to me that you might think that there are magic bottles of ink that are perfect for every tattoo and skin type. Don't be afraid to mix! In fact it is verrry often NECESSARY to mix colors in order to achieve the the right tone, hue, tint, or shade for a tattoo.
That being said, I too would like to purchase a good set of 4-5 pre-mixed grays. Just for time and efficiency, but mostly for consistency. You may need to consider having the exact same shade of gray for touch-ups sometimes. Until we find a good set I would suggest ordering some empty bottles and a big bottle of white and black. This probably needs to go w/o say but just in case it helps, and it sounds like it might, start by adding white to each of the bottles. Use a little bit less than you figure you'll need for each shade. Then add a lot less black than you think would be good initially until you get the right shade. Adding less gives you wiggle room to adjust the shades after they're all done. Afraid you won't be able to mix the correct shades??? Don't be! The Color Wheel really and truly is subjective. Just mix what you think looks good, then if you want to you can compare them to a color wheel or gray scale for adjustment. Depending on what it is you're doing, when you're tattooing you'll probably have to mix it in the cap or tube to get the right gray anyway.
Intenze does NOT make a set of grays, they only carry grey wash sets. They do have a "platinum" and "silver" which both appear like a slightly dark medium gray on their website w/ the platinum looking slightly warm and the silver slightly cool, but it's hard to tell on a screen. I've used the platinum a few times, but I had to tint it EVERY time. In person it's very dark, but very neutral (not warm or cool). Not a good medium gray though.
Now, back to the issue at hand, ha. I don't know who makes them if anybody. They really aren't necessary though. I've been a professional artist for 15 yrs and I've only been tattooing for a year and a half. It's been a long slow road until recently doing very simple tattoos and building my skills so it might not be my place to say this but I just gotta in the sincere hope that I've built some credibility by trying to help and that you listen. It's my opinion that whether you've been tattooing 2 weeks or 20 years, if you can't mix a set of grays w/o feeling like you're playing around w/ someones skin, you really aught not to be tattooing yet! I would strongly recommend starting w/ some paints and pencils and getting w/ a mentor for an apprenticeship before you "play around". This ain't a game! But if you continue down your chosen path I hope this helped a little.
Good luck Jay, let me know if you find a set! And remember, even if you find them the trick'll be getting them from a good company and learning how to use them.