I'm surprised by how few spells and magical items exist to make a humanoid character (other than the spellcasters themselves) appear like another specific humanoid character. Even wizards specializing in the Schools of Transmutation or Illusion don't seem to have any options for this.

For example, a character wishing to engage in espionage might use the illusion spell Disguise Self or the transmutation spell Alter Self to appear as a known member of the king's guard or royal council, but those are "Self" spells, so spellcasters would have to do the infiltrating themselves. They could not use it to disguise an ally. 

Other spells like Polymorph only allow the target to be turned into a beast, not a humanoid, so could not be used to impersonate a specific individual. The only spell I could find where a spellcaster of any class could shapechange the appearance of an ally into another humanoid, either by illusion or transmutation, was the 9th-level spell True Polymorph. Am I missing any lower-level spell options?

Similarly, I couldn't find any magic items that would do that. Even a Wand of Polymorph still only allows the user to cast the Polymorph spell with the spell's usual limitations of transforming the target into a beast, not a humanoid. Short of home brewing something like a "Cloak of Disguise" or a "Polyjuice Potion," are there magical items that would allow a character to impersonate a specific NPC?

Have I overlooked a spell or magical item that could do this?

The 5th level illusion spell: Seeming is what you are looking for, it lasts upto 8 hours, requires no concentration and you can make the tagets look like almost anythign you want. but other than that you might need the magical item:  hat of disguise.

Seeming does, ah, seem to be what you want. However, I'm a bit dubious of Hat of Disguise. It allows the wearer to cast Disguise Self, but uses the wearer's casting ability...a martial, perhaps a Barbarian or Fighter might not have a high enough Int, Wis or Cha to make it likely that they'd pass a check.

Thank you! I was using the “shapechange” tag to try to find likely candidates, and neither used it. Seeming is tagged with “Control” andHt of Disguise uses “deception,” which I didn’t think to consider. 

Quote from Linklite >>

Seeming does, ah, seem to be what you want. However, I'm a bit dubious of Hat of Disguise. It allows the wearer to cast Disguise Self, but uses the wearer's casting ability...a martial, perhaps a Barbarian or Fighter might not have a high enough Int, Wis or Cha to make it likely that they'd pass a check.

 Since a fighter or barbarian doesn’t have a spellcasting ability, it would only use their PB to generate the DC for the check.

I know you said magical but someone with a good deception ability and a disguise kit can do a great job as well.

I know this because I currently have a character in trouble with the law in Waterdeep and my rogue friend disguises me every day.  I wound up taking a level in Rogue to get proficient in deception to help him.

Quote from Linklite >>

Seeming does, ah, seem to be what you want. However, I'm a bit dubious of Hat of Disguise. It allows the wearer to cast Disguise Self, but uses the wearer's casting ability...a martial, perhaps a Barbarian or Fighter might not have a high enough Int, Wis or Cha to make it likely that they'd pass a check.

It's even worse than you think. By default if a magical item uses the wearer's casting stat for a DC, but the wearer is not a caster the DC is just 8 + Proficiency Bonus + nothing else.