Upload & Sell: Off
The lighting department at Glazer's (across the street from the main store, on 8th @ Republican) might have spare parts available, or at least a parts catalog with the stock numbers and could order for you.
If it's not something you want to do yourself, Photo-tronics (next door to Glazer's rental department on Dexter @ Mercer) is a good repair shop.
For a distributor contact, try:
Manfrotto Distribution USA, Inc.
P.O. Box 506
Ramsey, NJ 07446-0506
Phone: (201) 818-9500
Fax: (201) 818-9177
In case you haven't been to them before, the above named shops are:
430 8th Avenue North
Seattle, WA 98109-4705
513 Dexter Avenue North
Seattle, WA 98109-5284