Depends on exactly what you want and the size of hatches/storage spaces!
We have had a couple of cheap dry bags which lasted about 2 years before starting to leak, and an Ortlieb which has lasted three with no problems, so we replaced the cheap ones with another two Ortliebs.
http://www.ortlieb.de/_prod.php?lang...odukt=x-plorer
The rucsac style shoulder straps are excellent, and save a lot of struggling up ramps and slipways with your hands full of luggage and gear.
Stockists aren't particularly easy to find - we ended up getting them from a cycle shop in Manchester - I think it was here:
www.withingtoncycles.co.uk