The current end of the HT lead which connects to the current spark plugs is just a "push" fit on to the end of the spark plugs. This is a solid enough connection for it to work and should not give you any trouble unless it is "really loose".
You should "feel" it clicking as it is pushed on to the spark plug as the clip inside slides over the threads on the top of the spark plug.
Modern cars have the same technology (if slightly altered) in having longer plug caps or keeping the little top that is screwed on the top of new spark plugs. See picture below:-
![Champion L7.jpg](./download/file.php?id=2098&sid=f88db38d1d7e931d421f160d22ea3f69)
- Champion L7.jpg (10.29 KiB) Viewed 6843 times
The little silver top unscrews and then it will be the same as the ones you are taking out.
I have no idea where the best place is you can buy spark plugs from other than to say, your local car accessory shop or motor bike garage should be able to help you even if only to tell you where you could get the plugs from if they can't supply them. The champion L7 is a Haflinger recommended plug, so maybe they will cross reference it to what ever manufacturer they have dealing with, so you might end up with spark plugs from NGK or someone. As long as they are cross referenced to be equivalent to the L7, they should be all right to use.
John
p.s. if you are not sure whatch this:
http://www.youtube.com/watch?v=Zrww2THMoJg