18 mg is a decent amount if you can get good vapor density. You should feel a throat hit from it. Try to drip two drops on the atomizer wick (find and read a thread on how to "drip", set it down, and wait 30 seconds before putting the cart on. You need to wait to give the liquid a bit of time to wick down to the coil or you'll get that nasty taste.
Investigate on the forums and make sure you are getting liquid wicking down from the cart fast enough to keep the atomizer coil wet. Sometimes that can be a problem, particularly with the 510. What model are you using, it can help people to advise you if they know that.
anyway, myfreedomsmokes.com sells various strengths (up to 60mg, mixing only!) of unflavored eJuice at a reasonable price. I've purchased from them several times. And they have some deal about getting a free bottle of juice on orders over a certain amount. And you should search around the site and see if there are coupon codes available to forum members before ordering from any site, could save you a little cash!