The methods here are probably your best bet.
Reaction of the amine with methyl formate gives a formamide. Further reaction of the formamide with dimethyl carbonate gives a methyl carbamate, and pyrolysis gives an isocyanate. The resulting isocyanate can react with benzyl alcohol to give Cbz-protected amines or with tert
-butanol to give t-BOC protected amines. If you're using this for what I think you're using it for, t-BOC is what you're after, since it will stand up to any base you can throw at it, including butyl-lithium.
You might be able to get away with isopropanol instead of tert-butanol if you really can't get tert-butanol, but it's not really a suspicious chemical at all I don't think. t-butanol is also formed by decarboxylating beta-hydroxy beta-methylbutyrate aka HMB.
t-BOC anhydride isn't very suspicious either, but it could get suspicious fast if the rxn works.