Is this a trust administration or probate ordered administration? If a trust administration then I suggest you contact a stock broker who can give you a value. If this is a probate ordered administration, then the assigned probate referee can give you a value and their value is the one that will be binding on the estate value.
Have you been appointed administrator? If there was a will leaving the stock to you, you would be appointed executor through a probate court. If the total estate is valued at under $150,000 you can transfer them through a small estate affidavit, but the process is very laborous. If you choose to do this yourself, you can contact the transfer agents for each company for more information on the stock. If the total estate is over $150,000 and you are in a probate situation, the probate referee assigned by the court will and must value the stock on an Inventory & Appraisal that will get filed with the court.
First off all, a court appointed probate referee will be able to valuate assets of the estate. Sometimes referees overestimate or underestimate property values. For example, an overstated valuation of real property can be detrimental to the estate because the property will have to be sold at 90% of the probate referee's valuation or higher. In today's market even a slight error can cause a delay of administration of the estate as home buyers are harder to come by than before. As such, aside from probate referee's valuation you should do your own due diligence.
This can be achieved by hiring a specialist which in your case would be a stock broker. They will charge you a fee for doing it and you will most likely have incur the cost. However, a petition for reimbursement of expenses can be made with the probate court. It is within the court's discretion to allow or deny reimbursement. An invoice or a receipt from a stock broker for services rendered will help your request for reimbursement. For a better opinion it is best to consult with a probate attorney.