The “weight” of thread is very confusing for a person just getting started. The old system of WEIGHT is not very helpful. Most thread these days will be specified by a TEX number which is described as the number of grams for 1000 meters so the higher the weight in GRAMS the heavier (and thicker) the thread.
The “medium” thread in question (1oz/800yd) equates to a TEX 40. Most common thread in a store rack is Tex 30. Most likely you will have to order Tex 40 from some online source. Gutterman Mara 70 (Tex 40) is a very popular choice. One source ref: Waywak
If your hand-sewn project is not extensive you could just use “regular” thread from store rack and use two strands while stitching…thread the needle and knot ends together. Just make sure it is 100% polyester.
Happy Sewing