Here is a good site I have used in the past.

http://pro.ovh.net/~mesetiqu/allmylabels/jam.php

http://www.i-do-it-yourself.com/2009/06/for-the-lo.....ls-part-2/

 I used to just use Avery labels - rectangular ones 21 to the page. They are self adhesive and quite cheap to buy on Ebay and the code for them is J8160. Only slight drawback is they don't always sit very well on some of the more oval shaped jam jars so now I use round labels J8105 for these awkward shapes and for all the lids on my jam jars. It's so much easier to see what is in your jars if the label is on the lid. Really handy for when you are looking for a particular jam that is stacked on shelves or in boxes and also when selling them at fairs etc - you can see immediately from the top which is which.

I use the Avery DesignPro 5 software which you can download free. It is great because you can create any sort of design you like by inserting a pattern or picture to your label and it lets you start the printing anywhere on the label sheet so you don't end up wasting labels.....well worth a look.