Flat heated bed replacement for uneven bed
Several things to consider:


Nothing is perfectly flat.  It is flat within a tolerance.  The bed you have now is flat to within a tolerance.  If you want a flatter bed, you are going to have to specify how flat.
As @mac mentioned, aluminum is bendable; so, what once was flat can be made un-flat and the other way around.  Note: Your problem may not be your bed, it may be that the bed mounting frame has introduced stress that is bending the bed.
Because the heating element is applied to one side of the bed, the differential heating (between the top and the bottom) can cause the bed to bend at temperature; so, flatness could vary with temperature.


What to purchase:


The Anet A8 has a 220x220 bed.  A very popular size.  You can get replacements heated beds for as little as $20 from a lot of sources.
Because of the considerations above, rather than replacing the bed, you may want to consider attaching a 220mm x 220mm Borosilicate Glass Plate to the top of the bed.  These are also readily available (even from Amazon) for less than $20.  That is what I did.  The glass makes for a very flat surface and a great printing surface.  I attach mine with small binder clips.  That makes it easy to remove to take the print off or to clean when needed.