Over the years, I have spent a fortune on seam rippers. My main complaint is that they seem to become dull so quickly.
I'm really happy with the one I have now from Lee Valley Tools (leevalley.ca or leevalley.com) Lee Valley makes top quality tools for woodworkers, gardeners, etc.
To see a picture of this kit, on the home page, click on "Gifts" then Category "Arts and Crafts" pg. 2, although it's fun to browse through their amazing stuff.
quote from catalogue: Pro Seam-Ripper Kit
Developed in response to a customer who felt seamstresses and tailors were under-served in tool development, specifically regarding the U-shaped thumb stabber that is currently called a seam ripper. The Pro kit includes two sharp-end stitch-picker blades for starting a seam, and four safe-tip rippers that complete the job without snagging the fabric.
The instruction brochure covers straight and top-stitched seams as well as 3 and 4 thread serged seams, and the techniques you use for each kind. The six blades, two blade holders, and detailed instructions all come in a fitted metal case to keep everything together.
The blades are durable, but replacement blade packs are available for heavy-duty users. Suitable for anyone who does sewing, even on a casual basis.
Pro Seam-Ripper Kit
09A01.20 $18.50 (me: the 6 interchangable blades really give you the equivalent of 6 seam rippers; and, as they mention above, you can order replacement blades for the 2 handles.)
They're made in Canada, so there will be no duty to the US, and Lee Valley will not bill you until they send an you an email with the shipping charges and you OK everything.
If you are visiting Canada, just go to a store and pick one up.
For those of us who have physical limitations that make gardening difficult, I can't reccommend enough their folding kneeler stool (pg. 31 of the gardening catalogue). It folds down flat for storage. Used one way, it provides a comfortable seat; and, flipped over, it can be used as a padded kneeler with handles on either side at just the right height to push yourself up again.
I have had a spinal fusion and 2 total knee replacements and it enabled me to enjoy my garden and flower beds again.