TBH - and this is just my advice - I'm gonna advise against making merchandise for your comic at this point in time. Unless you've got a much larger fanbase elsewhere, the number of subscribers you have right now - although growing - and the fact you only have one open episode and one scheduled episode means you're probably not going to sell much at whatever comic con you're at, and you'll probably end up investing more than you'll profit (you might be doing it for the exposure, but in that case, a few prints and business cards is all you'll need; in the end, no matter how much you love what you do, conventions are for widening your fanbase and most of all, making money).
Most people in conventions make their own merchandise with stuff like button-makers. T-shirts are a little trickier, though. Main reason they make their own things is so they don't end up investing in a third party company and paying them a lot of money to make merchandise only to make so much when you try and sell it. It also gives you a lot more control over what your merchandise comes out looking like. Prints are easy to do if there is a store nearby in your neighborhood that does prints, either cardstock based or art print based (hell, I get easy prints done at UPS; they're not super high quality, but they still look good and they work for my situation where I'm just starting out).
Again though, at the stage you're at with your comic (unless you DO have a bigger fanbase elsewhere, in which case, good on you), don't invest a lot of money in merchandise right now. Go to a convention to widen your fanbase, hand out business cards, sell fan art prints that people will want to buy, do quick commissions/accept long term commissions, etc. Do some fan art and sell that to make some extra cash in your pocket. Don't expect to be walking out of there with hundreds of dollars in your pocket; conventions are a tough game to play, but when you play it properly, you can walk out with a few dollars more in profit over what you paid in investments.
