Pineapplefabrics.com has 118" wide backs. You can get them by the yard or a 3 yard pack. The 3 yard pack starts off at $37.99 or $13.99 per yard.

I'd suggest looking at sheets unless you find it acceptable to buy yardage and make some seams to get the size you want.

They have a limited selection of 120" wide backing fabrics but a better range of 108". I have been buying from then for many years too. Always happy with what I get!

Hancocks of Paducah has wide backing. Welcome to this fantastic board.

Wide backing
 

I second backside and fabric.com also https://www.heartbeatquilting.com/ca...20-inches.html. But I haven’t ordered from them.

I assume you mean the 108" backing. Some quilt shops sell it. Also, some longarmers sell it, just like they would sell batting. If you don't have either of those in your area, there are places online that have it such as fabric.com, Missouri Star, or Hancocks of Paducah.

I have never used it. I have just pieced my backings at this point, but I have seen it several places.

My go to for backing fabric is always Marshalls Dry Goods you can not beat their prices. Good luck and welcome to the board!

My apologies, I realize you mean the 120". Same answer...

Keep in mind that most backing will shrink quite a bit if you prewash. I've not ever tired using it without prewashing and then measuring how much the finished quilt shrunk up.

When I made a quilt for my king size bed, I wanted it to end up close to 120x120 when quilted and washed. I buy 108" wide backing and it usually shrinks up to around 102"-104", and then I trim off the selvages so end up with a bit less.

I have bought 120" wide "preshrunk" muslin, but it also shrunk up a bit in the wash, not near as much as regular quilt back fabric though.

piecing the back
 

When I have to piece the back I like to make a small version of my quilt squares or just use strips and put between the two cuts. Looks like you meant to do that!