Costs: Pin trading can be very expensive, especially if you buy pins after you get to Disney. Disney sells individual pins (don't buy these for trading) and starter/booster sets. Starter/Booster sets are usually sets of 7 or so pins for about $30. If you run out of pins or decide to start trading after you've arrived, booster/starter sets are your best bet. Other sources include the Disney Stores, Disney Outlets,
DisneyStore.com, etc. I would keep an eye on the DisneyStore.com's clearance section for good deals. You can also buy pins on eBay, but be careful. Those can often be scrappers/fakes, especially if they're cheap (e.g., less than a few dollars per pin).
Caveats: there are a few other things worth noting about trading.
Scrappers, Counterfeits, and Fakes: The second issue is scrappers, counterfeits, and fakes. For some folks this matters, for others it does not. You'll need to decide which camp you are in before you get heavily into pin trading. To the average pin trader, I don't think the difference between a scraper, fake, or counterfeit really matters (because none of these are legitimate pins), but there are slight differences.
Basically a scrapper is a pin that looks legitimate, but is not. It can be a factory overrun (i.e., the factory was contracted to make 1 million, but they made 1.1 million instead) or a pin that has some kind of error that should have been "scrapped". A counterfeit pin is a copy of legitimate pin made by someone that was not authorized by Disney. A fake is a pin design that was never authorized by Disney.
Unfortunately, it's not always easy to tell what pin designs are fake unless you know a lot about pins and what designs have been released. These pins can often look like legitimate designs and may contain all of the markings (e.g., copy right) to look legitimate, but they are not. You'll often be able to tell a scrapper/counterfeit because the colors are weird/off or the back stamp (the copy right) is wrong/missing/damaged. Others say you can tell because they feel "rough" or the edges are sharp/pronounced. Sadly, there is no sure fire way to tell a scrapper/counterfeit. It's worth noting that if you do end up trading with CM, the CM lanyards tend to be filled with scrappers, counterfeits, and (to a less extent) fakes. So if you really dislike scrappers/counterfeit, I would avoid trading with CMs. That's the crux of the argument: those that dislike scrappers/counterfeits feel they cheapen the trading and make it not worthwhile. Those that don't care, tend to say that they like the pin, so it doesn't really matter if it's a scrapper/counterfeit or not (they also say that if you can't tell, does it really matter?). Both points are valid.
eBay: eBay is a popular source for pins, but it can be a tough place to buy legitimate (i.e., non-scrapper) pins. The general rule of thumb is that you buy a "lot" of pins (e.g., 25-pins) and the price per pin is less than a couple of dollars each, you're likely buying scrappers. Also, if you buy pins that come in little baggies (instead of pin cards like the ones you see at the stores in WDW/DLR), those are more likely to be scrappers as well. My recommendation is to look for sellers that are selling pins still on their backing and/or unopened booster sets (also still on their cardboard backing wrapped in plastic). In truth, unless you only buy from Disney directly, it is hard to say whether you'll get scrappers or not.