I've gotten wide backing at Wide Fabrics and have been very satisfied with quality, selection, service, everything.
http://www.widefabrics.com/

Also, Craft Connection is having a sale on wide backing from 11/21 to 11/27, 15% off all in stock backings, if you can wait a bit. It's one of their ever changing weekly sales events.
http://www.craftconn.com/

I get quite a bit from there and also fabric.com. If you look around on fabic.com's backing section, there are some for $8.95/yd, 108" wide, free shipping. Can't beat that.

check out John Flynn's method for making a diagonal seam in th back. You can extend 45" fabric to 60" wide. Requires less than a yd additional in order to make it this way.

I use sheets.. and I found a thrift store that get new sheets in all the time... They get them from Kohls.. If you live on LI.. There are two Unique stores.. I bought a brand new king size set that was originally $17, and since it was a 50% off day, it cost $8.50.. My sister in law took the fitted sheet.. Now that I think of it.. I think Unique is also up in the Bronx..
Barri

I have purchased from this site also. I just received a wide backing from them this past week. Very nice. I will order from them again.

I buy from www.marshalldrygoods.com, their 108 inch wide is $7.99 a yard.

I just piece together larger pcs of my less fave fabric for the backs

I do that too, but I'm running out of those ;-)

Thousands of Bolts is one of my favorite sources for fabrics too! LQS fabrics at great prices.

I bought a pretty blue and white flowered backing from them recently and it was about 11.98/yd and I needed 3 yds so I got free shipping! The fabric was very nice.:o