Lord Vivec in vivec's palace has it you can't get it until way later though and by that i mean right before your battle with the main enemy Dagoth Ur he is tough though

i think if u kill viviec the dwarf in tel fyr can turn the item he has into wraithguard

after you become nervarine of the four tribes and Hortator of the three houses simply go
to Vivec and enter the high fane complex in the vivec temple and find the Archcannons
office he will give you the key to vivec's palace after you discuss temple doctrine with
him talk to vivec and discuss buisness then agree to take wraithgaurd and take the oath
and now you have wraithgaurd

lord vivic has wrath guard