Hi Allison,
The line item grid of your Sales Order screen has its own table, the SalesOrderLine table. You can add it to your report as a Detail Table and that should make it available for you. Take a look at our stock Invoice (Product) report to see an example of how this works.
Have you seen our tutorial videos for Report Designer? You can find them in our Help Center's Report Designer section here. They might be worth a watch if you haven't yet.
Hope this helps.
- Justin