Gasket changing (repair)

Check the owner's manual if you have one, for the type and size, or measure the width of the old gasket. Also measure the width of the channel where it will go.
Check to see if it is a rope gasket or flat gasket. Clean out the stove before taking the door off to prevent ashes from going everywhere, if it needs it.
It may be easier to lay the door on a flat surface, if you can get the door removed. Remove the old gasket and adhesive, you'll probably have to do some scraping, and make sure it's clean. If it comes out in one piece you can measure it to get an idea of the length. I'd lay the new gasket in place, and verify it fits and cut it to length. Apply the adhesive and install the gasket, and try not to stretch it. Give it a day before using it, to make sure it sets up.
I know the local big box stores have them, Rutland is a good brand, if they sell them.

One trick to cleaning out the channel where the gasket goes is to get a small wire brush that will attach on your power drill. It makes cleaning much faster and easier. And like double-d stated, don't stretch the gasket when you install it else it may go back to its original shape and you'll be left with a gap.

On the firebox door, there usually is a way to adjust the latch so it will be tighter or looser. Many times you can just adjust it to close tighter and not need a new gasket. Either way, replacing the gaskets are very simple to do. They have to be else I would not be able to do it....

Buy more than you think you might need. It bunches up or stretches apart a lot. There was a thread on here about gasket changing where someone talked about using a high-temp silicone instead of gasket cement to make it easier to clean out next time.

Agreed on the gatking door off hinges and laying flat, and using the wire brush. Also, even though it's extra work, I highly recommend dry fitting it with the ddoor back on the hinges before you cement it back in.

Also, it's going to be a little hard to shut at first--the gasket packs down to fit. Just don't overdo it--adjust your latch if you can for the first few days until it packs down.

Put some saran wrap or something between gasket and door frame while the cement is curing (overnight at least) so that the gasket doesn't inadvertently glue to the door frame. And cure it with the door latched.

First time is a learning experience. If it's warm and daylight and you have time to get it right--count yourself fortunate indeed.

It's very important to have the right size gasket. Too thin and the door won't seal properly. Too thick and the door won't close, it will pinch on the hinge side. Check the manual for size or measure the old gasket. The wire brush sounds like a good idea to clean the old gasket cement out of the channel. I usually use a screwdriver to chip out old cement. You don't need to get it all out, just the loose stuff and any thick buildups. Put a good amount of gasket cement in the groove, spread it around and press the new gasket in, then lock the door shut on it for 24 hrs. This presses the gasket into the cement and makes an impression before it drys. If there's fresh paint that hasn't cured on the stove it will stick to the gasket and pull it out when you open the door. The Saran wrap between the gasket and stove sounds like a good idea to prevent this, just take out before you light it. Here's a video that shows how to do it-

http://woodheatstoves.com/how-to-replace-a-door-gasket-on-a-wood-stove-p-12072.html

I tell my customers that unless they have the manufacturer spec on the gasket size to just bring in the door to our showroom. We'll size and cut the gasket and sell ya a tube of cement. You could probably find a local shop to do this so theres no mistake. Hell, some shops may be slow enough that if you have the door track cleaned out they'll glue the gasket in there for ya. I would anyhow. You may be lookin at $1.00-$1.50 per ft on the gasket and 4-5 bucks for the cement. So what, 15 bucks or so, you get to meet the local hearth shop and make sure its done right.

Good idea. I hope you have a good experience there. Let us know how it goes.