There are a number of suppliers of Guernsey 5-ply wool. This list does not pretend to be comprehensive, but consists of yarns I have actually used.
British Breeds Yarns (formerly Creskeld)
Comes in 100g balls. One of the main brands, though I’ve had mixed experiences (six or seven knots in one ball once). A bit difficult to track down at the moment. Available online from Iriss of Penzance, though I’ve never bought from them.
Available online from http://www.guernseywool.co.uk/ or direct by mail order from “Lamana”, White Cross, Cury, Helston, Cornwall TR12 7BG. Comes in 500g cones, nice and even to knit, with hardly any knots. Wide range of colours. Very friendly mail order service.
Frangipani is also available from the Yarn Cafe at http://www.theyarncafe.com in 100g and 500g cones and balls; they also stock a variety of other, non-gansey related knitting supplies.
This yarn doesn’t seem to be stocked widely (not that I go to yarn shops often!). The last batch I bought had “Wendy Poppleton’s Guernsey 5-ply” on the label, so I guess Wendy and Poppleton’s have merged. I tried the navy blue and found it knitted up well, and the colour had a nice sheen to it. It is available online – put “wendy guernsey 5-ply” into a search engine to find stockists, or go to the Iriss of Penzance website.
This is the website of the Hebridean Isles Trading Company, operating from the beautiful Isle of Skye. After a disastrous fire in December 2013, the company moved from Colonsay to Skye. They offer a range of products, including their own Colonsay wool, fleeces, knitwear, books, and courses. Bed and breakfast accommodation is located nearby. I haven’t tried their wool yet, but it’s next on my list. (You can also follow what they’re up to on their blog.)
Finally, here are some other suppliers you might like to consider. I haven’t used them myself, but if you read the comments below you might find recommendations.
Flamborough Marine – They’ve been around since 1981 and their website says they are, “Your source for authentic hand-knitted Ganseys, Gansey Knitting Kits, Armor Lux pure cotton Breton shirts plus a range of quality, pure wool, traditional knitwear from Le Tricoteur (the original Guernsey sweater) and Armor Lux of France.” Well worth checking out.
Bobbins of Whitby. Gansey kits (yarn and pattern) and gansey yarn in 1 kilo (2.5 lb) packs, in navy, atlantic blue, red and cream.