My personal recommendations may be a bit higher than those recommended by the canner manufacturers. They recommend nothing higher than 15,000 BTU/1500 watts.
There are several alternatives and there are some discussions here about them that include photos and links to models and sources. Some are electric and range from an stand alone, high watt (look for 1500 watt), counter top burner to installing a separate range top in the kitchen (or other location) devoted just to canning.
The more common alternative is gas (propane) burners. There is a wide range of gas burners, both single and double, available on the market but again BTU ratings are important. I would look for 20-25,000 BTU rating minimum and infinite heat control ability.
This burner http://www.amazon.com/Heavy-Duty-Sin.../dp/B0000AXAAA
comes highly recommended on some other canning forums. Primarily because it has such infinite control that the 35,000 BTU it is capable of can be easily controlled. One poster who has used it extensively for canning reports that only one - the smallest - of it's 3 burners is needed once it is up to pressure. So 15,000BTU may well be enough or something in the range of 20,000 BTU IF you have infinite controls.
The problems with so many of the gas burners available like those for turkey fryers is that they have no heat controls or only minimum control of the heat source.
As for electric the watts will all depend on what you want to spend for the burner. I have read good reviews on this 1300 watt one: http://www.amazon.com/Waring-SB30-13...sr=1-3-catcorr
but others report it takes forever to bring water in a canner to a boil and doesn't last long.
This one http://www.amazon.com/Cadco-PCR-1S-P.../dp/B00028X2Y6
at a much higher price also gets generally good reviews.
In general, a propane burner is going to be less expensive, more durable, and faster than electric.
Not to beat a dead horse but I can only stress that smooth/glass top stove manufacturers did not envision canning on them when they were developed. So if you bought one now you are stuck between a rock and a hard place and for that I am sorry.
Personally I think they assumed that no one practiced home canning any longer or that they were still doing it over a wood fire. Just goes to show how limited their thinking was at the time.
So if you are a home canner and you are considering a new stove, please eliminate smooth top stoves from your consideration. You won't regret doing so.
PS: Lucy - AFAIK
the recommendation is 15,000 not 12,000. Have you heard something different lately?