samuk wrote:I think a splitter box is what I need. Could anybody recommend one that would do what I’m looking for?
It is indeed -- although it is possible under some conditions to use a simple bodge Y-cable to split a mic's output, that arrangement is fraught with technical complications...
You'll normally need to break the screen connection on one output to avoid creating a ground-loop between the preamps, and be careful about sending phantom power from only one device. But as both preamps appear in parallel to the mic, there are issues with significantly reducing the preamp input impedance which can affect output level, transient distortion, phantom power voltage and more... So while it can be done, and it can work adequately in some situations, it's not a foolproof solution!
More reliable commercial mic splitters are available in two basic forms: passive and active, in single, dual and multi-channel formats, and with one, two, or many isolated outputs in addition to the primary 'pass-through' output.
There are pros and cons with both passive and active types, but in your situation a simple passive mic splitter would make most sense.
So, the standard passive mic splitter just contains a transformer of some form. The balanced mic input is passed directly to the primary output (which passes phantom power to the mic, if required) and is normally used to feed the 'main' mic preamp.
The mic input is also connected internally to the transformer, and that transformer provides one or more electrically isolated outputs to feed other destinations (but without loading the mic's output significantly, thereby avoiding most of the problems I mentioned above with the bogde Y-cable solution).
These separate transformer outputs often have individual ground-lift switches to 'cure' ground loops, and there may be an input pad option as well to avoid overloading the transformer with high-level mic sources. Note that the isolated outputs can't pass phantom power through to the mic.
In a typically live-sound application the primary output would feed the FOH console, and the secondary outputs would feed the monitor desk, location recording rig or whatever.
Passive mic splitters are available from a wide range of suppliers and in a wide range of prices. The main difference between the cheapest and most expensive versions (apart from the number of isolated outputs) is the quality of the transformer. Obviously, better quality transformers -- such as those from Jensen or Lundahl -- cost more than generic Chinese types... although whether you'll hear a significant difference depends on the kind of material, mics and monitoring that you're working with.
At the more affordable end of the range are splitters like the Studio Spares Red506 and ART ProSplit:https://www.studiospares.com/Microphones/Splitter-Combiners/Studiospares-RED506-Microphone-Splitter_458250.htmhttp://artproaudio.com/product/prosplit-transformer-isolated-mic-splitter/
While at the more expensive end are things like the Canford Splitter, Radial ProMS2, EMO E325, Radial JS2, and others...https://www.canford.co.uk/Products/21230/20-021_CANFORD-MICROPHONE-SPLITTER-1-channel-2-wayhttps://www.canford.co.uk/EMO-MICROPHONE-SPLITTERShttp://www.radialeng.com/product/js2http://www.radialeng.com/product/proms2
Personally, I'd recommend the Radial or EMO units which are high quality and will last a lifetime... The J series (with Jensen transformers) being particularly good (but very expensive). Having said that, though, I'm always impressed with the ART products when it comes to value for money, and I've used the 8-channel version of its ProSplit (the S8) with excellent results.