Here's an idea for window tracks: don't replace the track unless the track itself is shot. I just found teflon tape about 1/8" thick and 1/4" wide at a plastics supplier in my area (Reed Plastics, Rockville, MD). Using a sharp knife I was able to trim the tape to the width of my track and slide the tape under the glass. This is not the teflon tape used for plumbing to wrap pipe threads.. it's thicker, as noted above, 1/8". It cost me about $23 for a roll that weighed about 1 1/2 lbs -- Reed Plastics sold it by the pound -- don't ask why.
However. . . .if you still want track.. About 45 feet (black) costs $2/ft from Sima Marine in Eastlake, OH. I am not using it because I found the teflon tape will work for me and I don't have to replace all the track (my track is aluminum which isn't available any longer).
Window channels: it's a two person job to remove them and three persons to replace. Not that the panels are heavy, you just run out of hands while balancing hefty pieces of expensive glass.
You'll find just a few screws at the tops/bottoms/sides. On top, be on the lookout for a small bumper that prevents the non-moveable piece of glass from sliding around. It is held in place by one small screw.
After the screws are removed, you can fairly easily wiggle out the lower channel. A suction cup of the type sold to hold onto the hull while cleaning your boat from a dinghy (I think one brand is Holdtite) will come in handy to get a firm grip on the glass. You and partner lower the whole thing safely.
You may be able to get the unit back in place with two people -- I found a third person indispensable for just that few seconds where balance is required.
John Althouse of Marinette, Inc. sells the aluminum channels as well as the plastic ones and if you utilize aluminum, do it -- you will never have to replace them. Plastic channels will get killed by the sun after a few years. When things go back in place, use a good amount of caulk and you're done. One other thing you will want to check is that there are drain openings at both ends of the channels. A small 45-degree cut across the very end of the two upright, outer divider pieces will do that easily.
It's about a 15 to 30 minute removal job and allow the same to put things back in place plus a few minutes to cut the channels to the correct length.
One other thing, you may find that one or more screws are stuck in place. Drill them out using a very slow drill speed, or use a screw extractor tool for stuck screws. It is worth the $15 or so for the tool.