Simplest way to reduce voltage from ~48v to ~36v
You are able to use a fixed voltage drop of about 11 Vdc at about 120 mA.  This is fairly easy.







simulate this circuit &ndash; Schematic created using CircuitLab

The transistor is a Darlington device in a TO-220 package and has a reasonable gain of greater than 1000.  The Vbe drop is about 1.2V.  Choose the appropriate Zener diode for the desired voltage drop.  

The total power dissipation is about (11V) * (0.12A) or about 1.3 Watts.  You will need s small heatsink to keep the temperature rise reasonable.  A small heatsink from an old computer motherboard or CPU is a good choice.  Note that the transistor tab is connected directly to the incoming supply voltage.  Don't let it touch Ground.

Note that there is NO current limit and no other protection.  It's up to you to keep bad things from happening.  Do NOT allow the output to short to Ground.

[Edit]

From the comments:

1) First thought was to simply use a 5W Zener diode.  But those can be hard to come by these days.  Next easiest is to use a smaller Zener diode coupled with a buffer transistor - the Zener handles only a few milliwatts and the bulk of the heat comes from the transistor - it's easy to get increased surface area (small heatsink) and thus keep the transistor cool.

2) I'm simply mentioning that there isn't any current limit in this solution.  If there isn't any chance that you will "Oops" and touch the output to ground, then don't worry about it.

We can also substitute a LM317HV regulator for the transistor - this does have significant over-current and thermal protection.  But the transistor is less expensive and may be easier to get.  Mention in your comments if you want to explore using a LM317HV.

Do note that any of that transistor family will work: TIP120, TIP121, TIP122.  The transistor is dropping only about 11 Vdc.