In theory, mirror scrying can be successfully accomplished simply with any reflective surface. However as with many forms of divination, atmosphere is important. For some people the most atmospheric and effective scrying tool is a black mirror.
Perhaps the ultimate black mirror is an obsidian mirror. Such items are famed for their powers and famous seers such as Nostradamus and John Dee were reputed to use them.
Unfortunately true obsidian mirrors are expensive and hard to come by. Grinding and smoothing obsidian into a reflective mirror is a long process.
Some of the “obsidian” mirrors on the market are apparently obsidian only in the sense of being black. Whether or not this actually matters is something that only you can decide. Does your scrying mirror have to be genuine obsidian? Or is a black mirror from some other material just as effective for divination? If you decide that you really must have true obsidian then check carefully with the supplier before you buy.
Once you’ve bought your black scrying mirror then the techniques for use are much the same as those for crystal ball use.