So getting digitized copies can be an issue from some places. Some archives allow it, some don't. Most national archives will allow it and usually have the procedure for doing so posted on their websites. Obviously it's easier to get it done in person, but is possible to order them online.

The most important thing needed is be specific in what you want to order. Just saying mission reports from the 8th Air Force won't be enough. You will need to provide details of where they can find it using the finding aid for the documents and tell them specifically which ones you want. Some archival collections can be thousands of linear feet of material in hundreds of boxes for certain topics. Some boxes will be more useful than others so it's up to you to go through the material and find what you need, not the archivist. Beware some places do have a limit on daily allotments of what can be pulled to be viewed/printed, and will sometimes even limit you more on what can be printed if its online only. Some stuff, usually copyrighted material, but also donor stuff can also be refused to be printed.

Another important thing to remember is you will have to pay for the digitization. This varies in price from institution to institution and is usually cheaper doing digital than paper copies. However, it can still rack up to be a pretty penny if a lot of material is needed as scans can go from around 0.25-.40 cents a scan to over ten dollars based on the material. (Pricing information off the NARA Website https://www.archives.gov/research/order/fees)