As I get .into this hobby more and more I feel the need to make some simple repairs like replacing a mainspring. My understanding is that mainspring winders are necessary to do this job. Are these interchanable as to size watch? Do you need a different one for each size mainspring? Where does one go about buying some? You can see my level of repair ability is somewhere near Zero

Hey Mike, Mainspring winders are something that some use and some don't. Personally I find it easier and faster to just hook the spring to the outside of the barrel,and wind it in by hand. My jeweler says that he does not use them either. I change lots of mainsprings,and although I have a winder,I never use it. Let me add I am talking about pocket watches. I do not work on wristwatches,and those small mainsprings may require a winder. You can buy them from almost any jewelery supply or on Ebay. As for interchanging sizes,I don't know.

Mike, Disassembly, fitting (for proper barrel and arbor hookup), winding, loading to Barrel and installing mainsprings require three things;

Having The right NEW mainspring
Having the training and understanding to remove the Mainspring Barrel and correctly re-install same.
Having a Mainspring winder or knowing the method to "hand-wind" without causing a huge helical deformation of the mainspring that will ALWAYS mess up the Isochronism accuracy of the watch!

That all said, I use a mainspring winder because I found it far easier to install new springs without losing an eye or really messing up the new mainspring.

I suggest that you have it done by someone experienced with the task.

I bought my mainspring winder on ebay for $20, but I have seen them go higher. Its been one of the cheaper tools I have bought for this hobby. Its an old k&d hand held one and works just fine for replacing mainsprings. Here is a picture of one like I have.

Thanks Dave and Cecil, Dave I have had others replace mainsprings and believe I could do it after several attempts. Each time I do something new with a watch I get a sense of accomplishment and that's something worthwhile in an of itself. I'll give it a try and then probably send it to someone and have it fixed! I usually dont start with anything of real value, but, I do lean from the school of hard knocks.

Jim, That is the same winder I use. A great tool!