You could try this method. Reaction of your primary amine with hydroxymethylbenzotraizole (HMBt, which is dirt cheap) leads to an adduct. This is actually a type of Mannich reaction - where the three components are formally formaldehyde, benzotriazole (precondensed as HMBt) and your primary amine. By controlling stoichiomentry you can stop it easily at one addition.
The reaction conditions, from memory, is to mix HMBt and the primary amine in benzene with a trace of pTSOH, driving off the water with a Dean-Stark trap, or molecular sieves for small reactions. The adduct that forms can be recrystallised to purity. The the benzotriazole can be easily removed by borohydride in say methanol.
See: J. CHEM. SOC. PERKIN TRANS. I 1987, 805
The Chemistry of N-Substituted Benzotriazoles. Part 4.' A Novel and Versatile
Method for the Mono-N-alkylation of Aromatic and Heteroaromatic Amines
Alan R. Katritzky, Stanislaw Rachwal, and Bogumila Rachwal