I beg to differ. These are not as rare as you would think and thus, they are quite common. There is a huge difference between Ivan Alexander and Ivan Šišman. Most of the ones in the market place is the more common Ivan Alexander Grosh and that can easily be determined by the Monograms on the reverse, specifically the stylized M on the right. Ivan Šišman did not have a son named Michael. So, the original poster's coin is an Ivan Alexander Grosh. In regards to Ivan Šišman, those types are quite scarce and thus are difficult to find...
Bulgaria: Ivan Alexander (1331-1371) Grosh (Dimnik&Dobrinic-9.1.2)
Obv: Christ standing facing before seat, raising hands in benediction; IC XC and monograms across field.
Rev: Ivan Alexander and Michael Asen IV standing facing, each holding cross-tipped scepter, holding Christogram-tipped staff between them; monograms across field; stars flanking base of staff.
I should re-take the picture of this coin as lighting made the coin look a bit yellow:
Bulgaria: Ivan Alexander (1331-1371) Half Grosh (Raduchev & Zhekov 1.13.6)
Obv: Christ standing facing before seat, raising hands in benediction; IC XC and monograms across field.
Rev: Ivan Alexander and Michael Asen IV standing facing, each holding cross-tipped scepter, holding Christogram-tipped staff between them; monograms across field; stars flanking base of staff.
Here is an example of an Ivan Šišman Grosh as I currently don't have a picture of one. Notice the absence of any monograms..
and a variety with a monogram on the reverse, but no stylized M...
and finally, a collection of Bulgarian and Serbian Grosh that is waiting to be classified on my desk once I find my copy of Dochev...
A gallery of my coins and artifacts can been seen on
FORVM Ancient Coins