If you are thinking about making your own muslin swaddle blankets, then a good source for getting the material is Fabric.com
Fabric.com have a generic white colors for $4.49 per yard, which is a pretty good deal.
Let me know if anyone else finds a better deal.
Swaddle blankets are made of lightweight fabric like cotton muslin that is wonderful for helping to regulate an infant’s body temperature in contrast to thicker baby blankets on the market. Since cotton muslin uses a smaller weave pattern, it also allows for a much better stretch in the fabric to obtain that additional secure swaddle. Swaddle blankets are also often large (ie. 47”x47”). So with extra size and fabric you can properly wrap up your baby.