In some Beethoven works (e.g. 8th symphony - first movement) there are some parts with repeated sforzando. Are these intended as simple sforzando or sforzando within crescendo? Is there any historical source explaining this?
I'm actually not sure by what do you mean by "repeated sforzando". I guess you are referring to passages that are repetitive and that have frequent sforzandi (for example, a sforzando each bar, etc.).
In any case, there are no fixed standard rules for these things. You have to see the context in which it happens; in general, any interpretation will make sense if it is coherent with the context. See where the passage goes, where does it come from, what is the role in the structure of the music, what is the harmony doing, the character of the music...
It is difficult to generalize. However, Beethoven's music is known for its insistence. How to "insist"? You have to see. Listen to good recordings and try yourself. If the passages in question are steady in rythm, harmony etc. there may be no need to do crescendo. Some people whant to always do crescendo/diminuendo; well, truth is that music can sometimes stay flat. For example, people who is stubborn do/say the same thing many times no matter what; translated to music, a "stubborn" expression will not do crescendo/diminuendo, but just repeat the same thing many times (until at the end it will explode, or maybe exhaust, who knows).
In addition, Beethoven wrote in a quite meticulous way, so chances are that there is no crescendo if he didn't wrote it. Again, this is not always the case and nothing can be generalized. At the end, to bring a score alive, many things that are not explicitly written need to happen.
The sforzando symbol, sfz, is used to indicate a single accented note, much like the > mark, rather than a global change in dynamic level. As such, I don't see what the issue would be with repeating it.
As wikipedia explains:
Accented notes (notes to emphasize or play louder compared to surrounding notes) can be notated sforzando, sforzato, forzando or forzato (abbreviated sfz or fz) ("forcing" or "forced"). One particularly noteworthy use of forzando is in the second movement of Joseph Haydn's Surprise Symphony...
Sforzando (or sforzato or forzando or forzato), indicates a forceful accent and is abbreviated as sf, sfz or fz.
So in the example below, the off-beat sixteenths are meant to be accented:
To the second part of your question, while I'm sure there are period descriptions of dynamic markings, I am not sure what they are.
I don't know about the 8th symphony sforzando but the 5th symphony sforzando is pretty famous and pretty universally interpreted. Here is the Sforzando passage I am referring to(and I'm pretty sure the forte 1 measure early in the tympani is a typo from the publisher):
This is universally interpreted as having the first note of each bar accented while still doing a crescendo to forte. Can't say that sforzando after crescendo symbols always means accents in the crescendo but it does in this sforzando passage.