Entire sets being dropped on you is a fairly rare occurrence in game. The Novice Hall does it, and itll happen maybe 3 more times currently as you level up(all around level 50). Farming out dungeon sets and seeking crafted alternatives are fine if you want your gear to match. With that said, just using a mish-mash of the best stuff you have available typically works fine. If youre worried that your gear is behind then focus on the most important pieces first. Your weapon is far and away the biggest driver of your ability to cause and heal damage, so this is always the first thing to check. The next most important pieces are chest and legs, being worth a bit less then half what the weapon is to your overall improvement. The next tier down would be head gloves and boots(at roughly 3/5 the value of a chest or leg upgrade). Then belt, and lastly jewelry.

And finally, if having a matching set IS the most important thing to you(it is to me) then help is on the way. When you get to level 50 theres a quest you can do to unlock the Glamour functionality. This allows you the change the appearance of any visible piece of gear. So your collection may be from 4 or 5 different sources, but you can set up your glamours so they all look perfect together.