Just look online, they are pretty common these days.
Start here...
http://www.lowbrowcustoms.com/search-list/?q=risers Don't see something you like, check the manufacturers that advertise in The Horse magazine.
Be ready for sticker shock, you're looking at $120USD and up for new in most cases.
Another option, call the parts counter at Bent Bike in Auburn WA, they'll probably just tell you to come look, ask for Tony, tell him they used to be in the battery aisle region. Been a few years since I worked there...
Address: 4337 Auburn Way N, Auburn, WA 98002
Phone: (253) 854-5605