From a cinematic perspective, as mentioned earlier there are a number of material components readily available in a tavern or inn that she could pick up or touch during her act to cast a spell unobtrusively - grease, fat, salt, other edible items, possibly plants, etc. could be taken from a plate as she passed a table or plucked from a flowerpot, either palmed secretly or done in plain sight as part of the act.
Any sort of metallic/mineral component or gem could easily be sewn into or glued onto the costume without being visible to observers, as could a tiny bit of fur or even a bit of bone. Feathers could be worn in her hair, as could flowers, which could conceal other less flashy bits of vegetation like a small leaf or sprig of something.
It would make for a great scene to eventually have one of the party members discover some lost or discarded piece of the costume, and upon closer inspection notice all the odd things sewn into the lining: a scarf with small metal bits sewn into the end to give it some weight (so it swings better when moving) wouldn't be out of place for a dancer, but the fact that the weights are each made of a different metal - some common, some rare - would be.
Now, on the other hand, other components like a live spider or a bit of bat crap are going to be a whole different story, and pretty hard to explain, lol.