Ebay...there's a seller that usually has them in 11s & 14s but they're $9.99 for 3. Search for Singer 20x1...no spaces between 20, x, & 1. This place has them too but usually only in 16s...much better price!
http://www.sewingshop.com/singerindu...dles-20x1.aspx
The machines I have that use a 20x1 also sew fine with Boye 21s. My National Reversew (vibrating shuttle) can use 20x1s, Boye 21s, Boye 14s (slightly longer so won't work in SOME machines), or type BB or DD. And I can usually find a better buy on the Boyes or BB/DD (I just bought a pack of 25 BBs for 5.50!) ....also ebay. My Wards machine isn't picky either...they all work in it too. My Reversew can even use Boye 4s which are quite a bit longer but since it's a vibrating shuttle, there's nothing for them to hit...so I guess that's why they work. All the needles I listed are pretty much the same distance to the eye of the needle.