Here are some more sites. Always take care on the shipping costs for any overseas purchases, as the choice can make a big difference.
Firstly, as mentioned above, MCL -
Motorcycle Larry has great gear.
California Sport Touring, USA:
http://www.casporttouring.com/cst/motorcycles/Honda.htmlI've purchased quite a bit from these guys over the years - reliable and reasonable prices.
BikeQuip, UK:
http://www.st1300-accessories.co.uk/index.htmlPurchased gear here - some innovative stuff too. Rear shock protector, Fenda Extender,
MotorCycle Setup in Melbourne for
McCruise cruise control:
http://www.mccruise.com/A.M.H.P. Tamborine, Qld for headlight protectors:
http://www.amhp.com.au/Dale McLean Motorcycle Electronics for Starcom comms system and other goodies:
http://www.dmme.com.au/Fuzeblocks, USA. These are beautiful.
http://www.fuzeblocks.com/HeliBar risers. This is the manufacturer'ss site, to see all the models:
http://www.helibars.com/products-by-bike/hondaI purchased mine from California Sport Touring to save on shipping costs as I was buying multiple items at the time.
Good Wool Store, for a waterproof seat cover:
http://www.goodwool.com.au/m_products.htmDirect Line Parts, USA. I purchased a workshop manila from this mob years ago they still appear to have a long list of stuff for the ST1300 though.
http://www.directlineparts.com/store.asp?str=10Powerlet, USA: for lots of electrical goodies.
http://www.powerlet.com/productsYou can get these accessories through many other outlets too, so check shipping and get them with other gear to save.
Beaver Power, Japan? This Anglo guy has a good reputation and loads of nice gear really well made. haven't used him, but tempted.
http://www.easternbeaver.com/Main/Products/Socket_Kits/socket_kits.htmlRadguard, Coffs Harbour, for radiator guards (I've seen what a stone can do to a Honda radiator!
http://www.radguard.com.au/categories/Honda/David Silver Spares in the UK used to be a great store, but since July this year no longer deal in recent models (only Hondas built before 2002).