My grandpa started a custodial account for me through TD Ameritrade, I did not know anything about the stocks he purchased. I have now taken ownership of the account and have received a 1099-R. I could fill out the long and short term transactions just fine, but there is a section for undetermined transactions. I do not know when or for how much my grandpa purchased these and he has now passed. How do I go about finding the information needed to finish the form?

You need to do your best to find out the purchase date and based on that , then, you will able to determine the purchase price.The better you can determine the purchase date range - the more accurate would be the purchase price.
I am not sure however, why do not you try this website here; You may find the stock price from Sep 11, 2001 to Dec 31, 2001 here - CAL Historical Prices | Caleres, Inc. Common Stock Stock - Yahoo! Finance
You can also check under the ticker symbol in Yahoo. There is a place where they will show you price history. This will give you cost basis.
You need to do your best to find out the purchase date and based on that , then, you will able to determine the purchase price.The better you can determine the purchase date range - the more accurate would be the purchase price. You may also check under the ticker symbol in Yahoo. There is a place where they will show you price history. This will give you cost basis. ALSO the brokerage should have records