Onlinelabels.com is where I purchase all my mailing labels.  The prices are very reasonable & they have practically every shape & size you need!
I mainly use the 2 part shipping labels on which I can print postage.  Or sometimes I use Click N Ship.
They also offer custom size labels which is very diificult to find elsewhere.
Maestro label design software is a must, it's free simple & adds a professional look to your labels.  It's online so there is no need to download anything
Onlinelabels.com provides all your label needs, large or small. The website is informative, with tips, tutorials & much more.