I am trying to cast a resin block and hoping for very crisp sharp edges and corners.
Can you advise what kind of mold could I use? or is the way to get it is only by sanding?
You are on the right track. You have already mentioned the two ways you can get this done:
1. The mold. If the mold you use is made from a template with sharp edges, then those sharp edges will transfer into your casting.
2. Sanding. You can sand your pieces post-curing to give them the sharp edges you want.