There are a few "mods" out that are reasonably priced and perform very well. Personally, I am very fond of the eGo T Mod which takes a 18650 battery, has a replaceable power switch head (ignore any negative stuff you might see in reviews on YouTube...they are outdated. The control head is completely remodeled and is an excellent device), and the tube itself can be replaced to have different colors if you want. It has two power options...regulated at around 3.4 volt, or unregulated....which will give up to around 4.1 volts to start.
Electronic Cigarette - Batteries - Joye eGo-T MOD (click on the 20% off on the top left of the webpage for your discount)
Joyetech eGo-T Mod 18650 2200mAh Device - MyFreedomSmokes.com
If you want a mechanical mod, you can't go wrong with the Smoktech Telescope. It will take an array of different batteries and is really beautiful to behold.
This one is a new "kit" that Madvapes just came out with
Telescope Bundle
Or you can buy the telescope on it's own if you have everything else needed already
Electronic Cigarette - Starter Kits - Telescope (remember that discount button!)
Also here:
KGO batteries 1100mAh
If you want something unique that works really well and is perfect for "stealth"...the 3.7 Gripper is great. Takes a 18350 batttery.
Gripper
There are variable voltage Grippers there too...but they aren't as "stealthy" and I understand they are more "fragile".
Just a few ideas to start with.