Hi Declan,
Thank you for your update.
Query : I don't believe we currently have access to SfRangeSlider (I don't see it in my toolbox). What package does it come in and would we have to buy it?
In order to access the SfRangeSlider, you need to use the assemblies such as SfInput.WPF and SfShared.WPF in your project. However for RangeSliderControl, you need to refer the Tools.wpf packages of Syncfusion.
You can refer these package from the following link:
Query : I can check with our Support team exactly what licences we currently have but I suspect they'd rather we continue to use the controls we already have rather than purchasing something new. Is that possible?
You can use the RangeSliderControl , if you have installed with Syncfusion Essential build by searching it in ToolBox.
However since it is classic control, we suggest you to use our SfRangeSlider.
Please try this solution and let us know if it helps.
Regards,
Vijayalakshmi VR