Personally, if you're comfortable with soldering I'd recommend doing it yourself.
There are a couple of options you can do.
There are some bars in the stock that are a gold colour, you can solder your wire to the bars. These bars have a contact with the wire that runs up the stock tube. Think Eagle6 has a video on youtube for this, if not there must be a youtube video on this method.
My preference is to solder directly to the wires that run along the bottom of the stock tube and effectively cut the bars out of the system. This makes swapping stocks easier and I personally see it as another thing that could go wrong has been removed, including a reduction to the resistance of the whole circuit. The TM stocks also have the stock batteries partially visible as standard, so when you do the first method generally people have the wires partly on show or try to cover it with tape and it never seems to work.