A fetish perhaps? It really depends on the form that the potion takes though. Are they powders/liquids/ointments etc that get used up? Are they throw-away objects that get used up? Are they charged objects that can be recharged?
Also does your caster have a theme beside 'thaumaturgy/potion use'? When you say a 'Witch' do you mean they dance naked at the full moon? Are they a shaman? An ancient alchemist? Gypsy? Are they inspired by Morgana Le Fey? Hecate? Baba Yaga? Charmed?
You could create a modern variant that use dice, cards and poker chips as talismans to hold hexes to curse the targets fate. Imagine the fun narrating the various effects that hit your opponent varying from Final Destination to pure slapstick; slipping on banana peels, falling objects, hit by cars etc. Or perhaps a witch that uses an old polaroid camera to create impromptu voodoo dolls?
The "potion" idea is "an object enchanted to produce a particular effect just the once, for a short time or immediately, that can be throw away or retained for re-enchantment". So a coin soaked in oils, a stone (or brick!) carved with sigils, a metal ring twisted into a particular 'rune', a piece of quartz charged with electricity, and so on.
As for the definition of 'Witch', well, one of his aspects would be
Faux Wiccan - in that he isn't one, but people often think he is and he can pretend to be one if necessary (as described in his first novel, name TBC). While he may have been taught by more spiritual individuals (another aspect relates to them) he is much less spiritual about it, but they use the name 'Witch' so he does too. He's all about thaumaturgy, with the use of rituals and incantations in a Working designed to guide the mind down the proper path to release the gathered magical energy in the proper way. These rituals usually include the use of herbs, runic symbols, black or white candles, incense, crystals, and of course the symbolic link to the target.
I like the Polaroid voodoo doll idea