i looked on ebay for "singer sewing machine tensioner" and found this one for a 301. I have a 503a and it looks just like the 301, so maybe it would work on yours, too. i don't know but thought i'd at least pass it on.
http://www.ebay.com/itm/SINGER-301-3...item257a02bb1f
here's for a 329K
http://www.ebay.com/itm/SINGER-329K-...item232339fe2b
you could also message one of the posters on ebay that's selling parts of tension components (i saw several) and ask if they have that specific part. good luck!