Nixie tube display units are a quaint but somewhat popular technology from the past, and the market for tubes and accessories is still quite lively. However making your own is virtually impossible, so getting custom sizes or lettering can be a problem. However using different materials you can replicate your own tube in a way, and this has been demonstrated by Instructables member marty_vance.
His efforts use EL (electroluminescent wire) shaped into the form of the required characters to display, and are fitted inside a large glass jar. The rest of the EL driver circuitry can be fitted in the bottom of the jar or elsewhere, and when finished the results are quite good, for example:
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