I was searching and see that there are many people making their own strops.

I searched on Internet and found some leather straps, but I'm not sure if they can be used for strops, or if they need any treatment like sanding before use.

All of these leathers have these specs (according to seller):
Thickness: 3.0-3.5mm
Vegetable tanned full grain bridle / butt leather
There are colored (DYED THROUGH) and not colored
Seller is located in England

Here are some pics:
Name: $T2eC16NHJHoE9n3Kd234BRnlstIB8!~~60_12.JPG Views: 918 Size: 30.8 KB

Name: $T2eC16hHJF0E9nmFTM,nBROOvB,yhg~~60_12.JPG Views: 1605 Size: 18.2 KB

If they are ok for strops, I probably can make two strops, since I'm new if I damage one I can have another one ready