I buy Type A plastic-sided pre-wounds for 21¢ each here:
http://www.threadart.com/shop/category.aspx?catid=87
The description says there are 108 in a box, but they actually contain a gross. I buy a box of 144 when they're on sale for $30.
These work in my Singer 15 and my Kenmore 1803. Haven't tried them in my embroidery machine that takes 15J's, but it looks like the Type A bobbins are about 1/16" taller than the 5J's, so I wouldn't try them in that.
I love this poly thread - it's very soft and very strong. The only thing I don't like about it is that it's too shiny for some projects. :)
ThreadArt also carries Type L, paper-sided or plain-sided in black or white.