At a lab where I used to work, we would bring a left over bag of bread to 
work and place it in the cabinet and just leave it there.  When it had molded 
to the point where it had turned black, it was ready.  We would cut it into 
pieces and process in the same fashion as our tissue.  This worked every 
time.  I found that if you placed a slice of bread in a zip lock bag, the 
loss of oxygen would prevent it from molding, so for best results, just leave 
it in the package that it was purchased in at the store.  Good luck.  I hope 
this procedure works for you.

Jeanie Wade, H. T. (ASCP)