Devanagari calligraphy pen

“Ideally it should be 36 degrees” according to video Calligraphy Tools by Professor Santosh Kshrisagar.

“Left oblique” nibs are made for left-handers, and also for right-handed people doing calligraphy in Devanagari or Arabic.
from A Few Notes about Fountain Pens

In summary:

Nib should be left oblique
Angle should be 36 degrees
It may be necessary to purchase a foutain pen with a ball-point nib and cut it to 36 deg and then sharpen it with fine sandpaper
Maybe keep checking around for calligraphy pens at stationery stores because someone might have something

According to a Sanskrit professor and author:

Devanāgarī is the common script used for writing many Indian languages [much like roman is used for most western languages] and as such one can write it with basically any writing tool or one can download type-scripts for your computer. You are, I believe referring to the script you see in many manuscripts, and the stylus’s used for those were traditionally hand made. I am not sure that these days that there are any special ones available, even in India.