You can certainly buy commercial slip rings to send power from a yawing windmill down the tower. But slip rings rated at 100 amps at 12vdc would be very expensive.
There are plans out there on the internet and in magazines for homemade slip rings. Most use carbon brushes and copper pipe, insulated by PVC pipe. Brushes from car alternators can be used. An example:
http://www.fieldlines.com/story/2003/5/8/55448/19618
and another example:
http://www.fieldlines.com/story/2003/5/8/154641/0612
DanB and DanF at Otherpower don't use use slip rings. We've always used a "pendant cable," which twists inside the tower. We also discussed this issue with homemade wind power guru Hugh Piggott... he also uses a pendant cable inside the tower pole. We looked at a commercial AWP wind turbine install on a 120 foot tower that uses a pendant cable.
Though for most folks it's hard to believe at first (until they try it), problems are rare with a pendant cable. A properly-furling wind turbine won't rotate much. We've never seen one do a 360 turn in less than a week. We put an electrical plug and socket at the bottom of the tower, and have found it necessary to unplug and untwist the pendant cable only once or twice a year. In a more turbulent environment, you should check and untwist it more freuqnetly. We also shut down our mills using this plug -- we plug it into a shorting block to shut down the mill for maintainence or during high wind events.
Don't run the pendant cable so it wraps around the outside of the tower, and don't try to make any sort of 'stop' for it--both of these could prevent the turbine from furling!
Use the best quality flexible stranded extension cord wire you can find, and be sure to use a kellum grip or other strain relief at the tower top, where the wire goes through, so the weight of the wire doesn't pull on the stator.
Discussion on slip rings vs. pendant cable:
http://www.fieldlines.com/story/2003/5/8/55448/19618
