You can calculate the average of the market high and low on the date of the donation to determine the average trading price.
If there is an active market for the donated stocks, the average trading price is the average price between the highest and lowest quoted selling prices on the valuation date.
For example, if the highest selling price for a share was $11 and the lowest $9, the average price is $10. You get the average price by adding $11 and $9 and dividing the sum by 2.
You can use online resources like Yahoo Finance, the company's web site, or similar to get the stock's high and low price for that day.
If the stock you donated is not publicly traded, you will need to do a little more research. Click here for more information.