It is common for a rider to request a specific brand and model the provider does not have. In many cases an equivalent exists that serves the same function and sits in the same tonal range.
How to raise it
State which model the artist requests and whether they accept alternatives. A provider can propose gear with similar characteristics, and the artist or their technician decides whether it is acceptable before confirming.
What to check
Amplification type (tubes or solid state), cabinet configuration, number of keys or action type on keyboards, and compatibility with the rest of the setup.