A lot of people struggle filling up the word count but I went over it at first because I focused too much on content so its important to remember your argument is just as important as evidence and not to overwhelm your answer trying to get as many facts as possible in.
The source is contemporary of the period so like you said can be a diary or speech etc and acts as evidence but can also link to your argument. Sources aren't going to be historical opinions like an interpretation so won't necessarily support your argument. Like I've used Ida B Wells speech about lynching but that doesn't really support my argument. I used it to link in to one of my points (which was that because of the importance of key black figures after Reconstruction ended, it wasn't necessarily accurate to say Federal Government inaction was the main barrier in the advancement of Civil Rights because this was the period most lacking Federal support and yet the movement advanced quickly due to the work of people and organisations like Ida B Wells and Booker T Washington - and later the NAACP and Malcolm X) and then used the speech to identify Ida B Wells as one of these figures and to show her influence. Its not my best use of a source but theyre basically used to back up a point - not necessarily your whole line of argument. You need a balanced argument so it's good to have them to either support a point that leads into your LoA, like I used the speech I talked about, or even a different point that disagrees with your LoA.
If you're doing AQA A-Level, then you probably have exam questions about extracts and sources (30 markers). Extracts basically are historical interpretation's because it's a historian's interpretation of history. It's ideal to have an extract that shows the historian's opinion and should be differing from each other but not entirely contrasting.
I haven't heard of the sources you talked about so check with your teachers instead because they can tell you whether they fit the right criteria or not. I'm not in love with the idea of using Yale lectures as sources (I think you said? If interpretations, thats slightly different) but even so, I think you need to find a specific part of the lecture to use. Your sources need to be contemporary of the period but your interpretations can be relatively recent so if that's the case, I don't see any problems.
I think books are still more useful to use, you only have to cite them so you don't have to include a whole interpretation or read the whole thing. Your teacher should have some to relate to the topic and you can look through them and look at the contents page to see where there might be something relevant to your point or argument and then you can look over that section. Books are useful for interpretations and I think you might struggle to find one that works that isn't from a book considering most historians publish their opinion through books or journals.
I added my interpretations in to show my own argument and how historians agree with it but it's easier to view sources and interpretations as evidence to support your argument and there is nowhere specific for them to go.
I would say sources are more evidence to support your argument/point/factor and interpretations support your judgement.
I think it is better to focus on the knowledge as a whole as once you know the actual history of Civil Rights, you can make your overall judgement and then choose factors to support your argument.
I found my sources on the loc.gov website and in the book African American voices (which is a coalition of sources like speeches, newspaper articles etc). One is an image, one is a newspaper article and the third is a speech.
Both my interpretations are from books and Better Day Coming is one I recommend highly. Everyone in my class loves it so I think it could relate well to your question.
There aren't any specific websites I found that became integral to my NEA other than loc.gov but maybe look on google scholar for articles, they might be useful. I also suggest researching after you learn in lessons because there is additional information that may not be in the book and you could find some good quotes or a cited interpretation. A good website is Miller Center but it is slightly biased so keep that in mind.
At this point in time I think you should focus on learning and doing extra reading outside of lesson to find out as much as you can because once you have all the knowledge you can identify your own overall argument and judgement and then factors to include in your NEA so it will be easier to find relevant sources and interpretations.