Buy either on Craigslist locally (so you can make sure it works) or buy from a site like the aformentioned Adorama.com (there are several other options like it out there, but I just know from people that have purchased used equip, including myself, that they have some of the best customer service you'll ever deal with, and ship quick and fast - and they are pretty spot on with how they grade their used gear, so you don't have to cross your fingers much)
Ebay and Amazon are alright, and there are TONS of things on there - but you're usually hoping that someone posted the right photo, and hoping everything works as well as they say it does - at least from Adorama or something, you have a bit of a guarantee.
As for lenses, I'm just going to put this out there since I own both and collectively the cost me around 150 bucks total - get a 28-80mm 1:3.5-5.6 V USM kit lens from the late 1990's. It's one of the best kit lenses around from my experience, and has some of the smoothest and quietest autofocus since it's an Ultrasonic Motor - which is more accurate and quiet than other lenses (it might be one of the most affordable Ultrasonic Motor lenses from Canon). It's slow-ish as far as letting light in, but for outdoor stuff, it will cover most of the focal lengths you would have to deal with, and it's sharper than most other lenses. It will run you around 60-70$ used.
I'm not 100% positive if Nikon or Pentax have anything equivalent, but for my pentax camera I just use old manual lenses from the 70's and 80's that come good and cheap, and I'm Nikon-retarded for the most part.
And then, as every photographer should have, you should get a fast prime lens like a fixed 50mm or a fixed 35mm. Both are very practical, and will let in more light than any kit lens - so they will be better for night photography, or anything indoors. You can find a Canon or Nikon 50mm f1.8 for around 80-100 used. A new one will cost around 120 or so from either company, and they'll both be solid. Pentax's 50mm f1.7 is slightly faster, and maybe a little bit better quality? but i'd just recommend finding an older version of the same lens for around 50-60 bucks that's manual focus.
For 35mm primes, it's pretty much the same thing - only each company's lens will run you closer to 150$ rather than 100$ for the 50mm.
sparknotes? Get a decent kit lens with faster autofocus, that allows more light in than standard stuff, and a fast 50mm or 35mm prime and you'll be set for having a few lenses.