To be "straight from the horse's mouth" is to be from the most reliable source.
I am wanting a title like "The horse's mouth" for my newsletter but I don't really want to call myself a horse. What other idioms are there that I could use?
A humorous way to refer to oneself in speech or writing, particularly when you want to emphasize your own situation or role, is yours truly.
From McGraw-Hill's Dictionary of American Slang and Colloquial Expressions, for example:
n. me, the speaker or writer.
If it was up to yours truly, there wouldn’t be any such problem.
or from the Cambridge Dictionary of American Idioms:
Even though he never graduated from high school, his business ability rivaled anyone's, yours truly included.
Some folks, such as yours truly, can't resist a clever pun or play on words.
Usage notes: usually used as a humorous way of referring to yourself
Endangered Phrases by Steven D. Price (2011) has a little more detail on its origins and register:
For whatever reason of modesty (or false modesty) that prevented speakers or writers from using the first-person singular pronoun “I,” the “yours truly” convention was established. It came from the standard letter closing. It sounded mannered when it was first used in the 19th century and even more so now. Other equally stilted circumlocutions for “I” or “me” used in writing are “your reporter” (still found in alumni class notes) and “your correspondent.”
All definitions located via The Free Dictionary.
An original source of something (Oxford)
If you're okay with calling yourself a fountain, that is.
You could have the inside scoop...
the scoop US, informal : information about something that is currently important or happening or that is interesting to many people
* Did you talk to him? What's the scoop? [=what did he say?; what is happening?]
* I was with them when it happened, so I've got the inside scoop. [=information known only by a particular group of people]
also, from LearnEnglishNow:
What does "inside scoop" mean?
Definition: to be knowledgeable; know something most people don't
4 American English informal: information about something:
the inside scoop (=special information that other people do not have) on the markets