I have a leak in the seam of my old baseboard heater. I don't want to replace the whole baseboard. Is there something I can use to get it sealed through the winter without using sodering it?

Sorry, but with hot water heating baseboards, if there is a leak on a solder joint, it will need to be repaired by way of soldering. Just not any quick fixes here. I would suggest having a pro come it to take care of this, as they will know the most efficient way of doing it.