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Hi John,
You're quite welcome. If you're looking for papers containing keys to North American diptera, BugGuide.net is a good place to start (use the "Links" and "Books" tabs under the taxa of interest). There'll be URLs to the papers there although many recent papers will be behind paywalls. If the link is to JSTOR (https://www.jstor...), you can get free access to 6 papers per month by setting up an account. Access to older journal volumes at sites like the Biodiversity Heritage Library (https://www.biodi...ibrary.org) is free.
The Manual of Nearctic Diptera has many keys although some may not be up to the latest taxonomy (http://esc-sec.ca...ions/aafc/). It also contains a list of reference papers for each family.
The Diptera volume of Nomina Insecta Nearctica (http://nearctica....olume3.htm) contains current names of taxa and their synonyms along with author last names and dates. Using that info in a web search sometimes turns up an accessible paper.
The Canadian Journal of Arthropod Identification (http://cjai.biolo...urnal.html) has some excellent keys to some dipteran taxa, both as PDFs and as online keys. The geographic range of those keys in limited to northern North America but the keys I've used have been excellent.
I hope that helps

Best wishes
Steve