That's fairly easy. If you have sturdy fabric, thread, those sewing needles, scissors, and a brain, you can make one yourself. You'll need to take the calc and sorta fit the fabric around it.  Unfortunately, I can't help you with the fabric because I have no clue what you need. 
If you're not the creative type, there are plenty neat holster like things available on the web.

Or just put it in your pocket.

I've tried a lot of similar crafts. If you don't mind it being a bit sticky, the easiest way would be to make it out of duct-tape like you would a duct-tape wallet. Or, if you have a strong fabric like canvas or denim (maybe cut up a pair of jeans), you could stitch a holster together that probably wouldn't be too bad.
And, of course, you could go all out and purchase leather from a craft store and sew a high quality holster.

Quote from: The_King on March 29, 2014, 09:41:10 pm

Quote from: aeTIos on March 29, 2014, 05:18:39 am

Or just put it in your pocket.

that is probably the best thing to do anyhow

That.

True, calc holsters aren't very popular right now