All I can say is I've been growing cannas for years and years, and I've bought the corms from all kinds of different sources, and never had any affected by viruses ( or that have had their growth affected).
Possibly virus free ones have more vigour, but I'd guess this might just be a marketing ploy. The only way they would get virus free stock is by micropropagation techniques producing the plants from meristem culture, as the few cells in the growing tip are apparently free of virus.
I'd just buy your cannas for the best price you can find, of the colours and types you want.
Incidentally, lots of plant features such as variegated leaves, pale leaves and so on are often caused by viruses anyway, as almost every plant is affected by viruses.
Viruses of all kinds are thought to have first developed on or in plants in evolutionary terms. But few are actually harmful to the plant.

If you are worried about viruses, you can grow them from seed. I tried it when the white ones came out, followed the instructions on the package, and got 100% germination. They also flowered the first year. Viruses will spread through root stock and bulbs, but not seed.

Thanks Bf and W, I only became concerned as there was a lot when googled about a virus that wiped out many canna although some did manage to shrug it off. In the pics the leaves were badly affected as were the flowers and the only thing to do was to burn them or dispose of them via municipal bin. It was only one specific variety of canna that they were offering as virus free Bertie and yes it had been micropropagated, but it made me think that their other stock might harbour the virus (paranoia again lol) I suppose the wisest course would be to buy one and see how it goes. I love the ref flowered ones, but think it will have to be the orange flowered stipey leaved one whose name escapes me at the moment, not striatum. Monty Don seems to find it vulgar:-) I would love to have the facilites Wylieinthea to grow from seed, but unfortunately I dont:-)

All it takes is a heating mat, I used old cottage cheese containers, some dirt I enriched with extra perlite, and protection from the cats (they love the heat). With the heating mats, a lot is possible and they sell small kits.

I'll look into it, thanks Wylieinthea:-)

I've got several Cannas as corms bought from various places (cheap places and posh ones!) and have never had a problem with virus.

This is all very reassuring thanks Bamboo:-) I wonder what they were going on about???

Well I know there is a virus that can affect Canna corms/bulbs, but the seller is probably making much of it in order to sell you a much more expensive "virus free" version - if they're selling Cannas, they're not exactly impartial, are they, lol

As said above, cannas are very easy from seed and often will flower the first year from seed if started early. Being a skinflint, I wander around the town flowerbed displays in autumn and collect seed from the plants there (bit warmer in France so they ripen quite early.) I have a nice batch to start off this year, and there's always a chance that a few will come true from seed.

Will try seed maybe when more experienced with them. I have ordered a durban (tropicana) black knight and red king humbert in 5 litre pots and very reasonable too so fingers crossed. Should they go straight to my garage ?

While the weather is still freezing, just put them in the garage as you suggest. As they are already potted up, you don't really need to do anything else but put them out in the spring once the shoots begin to come through. Don't give them more than the absolute minimum of water until the weather warms up, as it's easy to make the tubers rot in a pot while they are dormant (I've done it too often!)
Black Knight is a beautiful variety, but I don't know Red King Humbert. Good luck with your cannas. Perhaps you can post some photos when they are in flower.

I certainly will Bf, can't wait:-)

And I'd like to know where you ordered them from, Bornagain, please? I have Durban already, but the Black Knight I'm interested in

Hi Bamboo, the company is Vale Exotics They're in Evesham, Worcs. A family firm. The plants were £5.99 each which seems very cheap compared to other sites. I have only just ordered so don't know about the quality yet, but they do seem to have lots of exciting exotics:-)

I hope they're online, about to check it out now, thanks for that

Just checked them out, Bornagain, looks interesting - but I would just say if you want to see a pic of what Durban actually looks like, have a look in my photos section - there's a pic of not just the flower, but one of the leaves too. The first year I grew it, it had a lot more yellow in the leaves, though. The pic they have on the site has been "touched up" I'd say...

Looking now Bamboo, I must say I thought their durban looked odd, a bit off putting:-)

My cannas arrived today, how's that for service:-) They look very good, one has a 3mm over one inch diameter stem or should say stump. I'm sure they're fine:-)

Ssshh, don't tell anyone, but mine are still out on the balcony, looking very, very nasty and sorry for themselves indeed after all this bad weather...

Shan't say a word:-)