How to compute the TF-IDF scores for a handful of documents, but without my own corpus?
Answer

What do you value in a ranking function? If you don't value TF or IDF, then it'll be difficult for you to achieve what you want to achieve.

Without a corpus however, you could use a frequency chart as a substitute for TF:

http://www.wordfrequency.info/free.asp?s=y

I had trouble finding a similar list for IDF online. However, you could just remove the IDF term from the TF-IDF equation. Finding an alternative for IDF is difficult.

Your Situation

4-5 documents isn't much, but it's still a corpus. Of course, it'll likely have more error than a larger corpus, but I see no reason why you can't use it. Also, why can't you use IDF? If the term exists in one document, then IDF can be computed, otherwise it shouldn't be in your controlled vocabulary anyway. Although if you're worried about error, then you could probably just discard it at this point.

If you just use your documents as the corpus, you can take advantage their context, which seems to be what you want here. Basically, many words will be more/less common in your subset than in the English language in general due to the specific domain and small sample size.