Typical nitrones sources are hydroxylamine oxidation or condensation with carbonyl compounds. Secondary hydroxylamines oxidize to nitrones in air over a timescale of several weeks, a process cupric salts accelerate. The most general reagent used for the oxidation of hydroxylamines is aqueous mercury(II) oxide: 
However, a hydroxylamine with two α hydrogens may unsaturate on either side. Carbonyl condensation avoids this ambiguity...
...but is inhibited if both ketone substituents are bulky.
In principle, N-alkylation could produce nitrones from oximes, but in practice electrophiles typically perform a mixture of N- and O-attack.