Use a shilling, and bobs your uncle (sorry!)
More helpfully a florin (two shillings or two bob) or half a crown (two shillings and sixpence) might be better as shillings were quite small coins
An old penny was quite a large coin and would have a certain charm
None should be at all expensive, they were mass produced
( British slang explanation: a shilling was called a bob and Bobs your uncle is British slang for there you go. Also a two bob bit is Cockney rhyming slang for, well, a Brad Pitt)