A replacement 21" blade for an Echo 58-volt lawn mower (CLM-58V series) can be hard to find. Echo lists the part number as 681938001, for mower models CLM-58V4AHCV, CLM-58V4AH and CLM-58VBT.
Gardner Inc sells that blade for $21.99. Jack's Small Engines lists it for $19.99, but describes it as a Homelite blade (though it's the correct part number).
According to Home Depot, ALB-58V is also a valid part number for that blade, and they sell it for $19.97. Handy Store also sells that part number, for the same $19.97 price.
Outside of those four vendors, you might not have much luck finding a replacement blade. Amazon doesn't currently offer it (under either part number) and eReplacementParts has a listing for it, but says it's "not available for retail sale."