I doubt that you will be able to order exactly the same muslin online because there is such a variety of weights, thread counts, and manufacturers of muslin. Even if you take it to the store, without the SKU you will probably have great difficulty matching your sample exactly.
I like Kaufman Kona muslin and buy it online -- usually from Hancock's of Paducah. They seem to put it on sale sometime around Christmas at a pretty good reduction; seems I have gotten it for half price at that time (sometimes with free shipping).
You may need to make a trip to JoAnn's in order to see exactly which muslin fabrics you like. Once you know manufacturer, brand, color, etc. it's pretty safe to order it online. Without that information, however, it's a big guess. In that case I would stick with recommendations from this board. Just be aware that thread count, color, and finish could be different from what you have been using.