Publicly traded companies provide an investor relations or investor services page on their website. Usually you can find a link to these pages on the company's home page.
Or you can search the company name plus "investor relations" using your favorite search engine.
These pages often include a "historical pricing" section or something very similar. Sometimes the pricing information does not include the dates you need so you may have to go elsewhere to verify the price.
Other information you can locate on these pages includes:
- Stock split information
- Dividend payouts
- Company's most recent SEC filings
- Press releases