This forum has a "second hand" section. That is a good place to start, but you'll probably be dealing with someone far away, so I suppose it is possible to get ripped off. You can also try craigslist or ebay, or look for a local kite forum.
Before buying a kite, figure out what size you need. Different local conditions require different sizes. 10 and 14 would be a good quiver for many places, but if you have very low wind or very high wind, it wouldn't work so well. Kite size is also dependent on weight, if you are unusually heavy or unusually light, you'll have to adjust for that.
Best brand kites have some good options for beginners (cheap, easy relaunch, good wind range). You can sometimes find new kites of last year's model for very little money. They also tend to show up used a lot. I'd look for kahoona or waroo. If you are on a tight budget, you are definitely better off getting one good kite than two not-so-good kites, and you can get a good kite used for $400-500 pretty easily.
There were a few Best waroos on the second hand section of this forum for really cheap, those could definitely work. There were also some newer best kahoonas that could be really good kites for you if they are in your budget.
I just went to toronto craigslist and typed in "kite" (I don't actually know if you are close to toronto or not...) but here is something that could be great for you (waroo and kahoona):http://toronto.en.craigslist.ca/yrk/sgd/3009937307.html
I don't know the person selling it, so I can't say whether the kites are in good condition or not, but that would be something t.o consider. If I had to design a kite quiver for a beginner on a tight budget, it would be pretty close to this
The US ebay has many more options for kites if you can find someone to ship out to you.
It sounds like you are taking lessons, I'd definitely talk to the instructor about used gear, the schools will often sell their used stuff pretty cheap, if not, they can probably point you in the right direction of where to look.
Also, regarding the trainer kite: yes, buy one. If at all possible, try to log at least a few hours on a trainer prior to the first lesson. Lessons are expensive and coming prepared (having some experience with a kite) will allow you to get much more out of it. When I lived in the mid-west I would use trainer kites with ice skates in the winter, which is a really good winter activity.