One of my favorite source docs is the Austro-Hungarian Red Book, that is, their official collection of diplomatic telegrams. The telegrams, covering the period from 29 June, 1914 all the way through to 24 August, 1914. Topics covered range from instructions for and reports about the investigations in Serbia, the ultimatums, as well as the international wrangling between the Great Powers as sides were staked out and intentions made clear. The vast majority of the telegrams come from the period of 24 July and 1 August, and show how Austria-Hungary tried to justify its war on Serbia to the other nations, as well as the international reaction to the Austrian desire for war with Serbia over the assassination. It highlights the lack of cooperation between Belgrade and Vienna, as well as the pan-European sympathy and general outrage at a very personal level by many of the players in the lead-up to war.

Of course this is a whole collection of 69 distinct telegrams, to and from, so it isn't a singular source, though it's certainly a very good starting point. If I may though, if you're looking for a single source, or a particular subset of these telegrams to focus and build a portion of the lesson around:

Telegram 7 (Berchtold to von Giesl, in Belgrade), sent on 22 July from Vienna, is a particularly interesting one for the class, as it contains in it the ten point ultimatum that Vienna sent to Belgrade as part of its forty-eight hour ultimatum. Most secondary sources I've seen reproduce the demands simply as a list - the ultimatum, in and of itself, is readily available anywhere and everywhere it seems. However, I have found that you gain a new appreciation for the mindset of Vienna when you read the list of demands in its full context, as but part of the whole telegram.

Telegram 8 (Berchtold to the Ambassadors in Berlin, Rome, Paris, London, St Petersburg, and Constantinople), sent also on 22 July, from Vienna, as well. In it, Berchtold gives warning to the Imperial and Royal Ambassadors stationed in Germany, Italy, France, Britain, and Russia that an ultimatum has been passed on to the Serbian government. He places in there a rationale for the decision to send an ultimatum as well as what could be considered as talking points on how to defend the Austro-Hungarian decision. It insists that Austria-Hungary wants good relations with its smaller neighbor, and that it is only making such demands, which are entirely reasonable given the circumstances, because Serbia is refusing to cooperate and do its part in getting to the bottom of the assassination of Franz Ferdinand. Most tellingly here, it reminds all of the parties that Austria-Hungary, nor any other nation, can "permit regicide to become a weapon to be employed with impunity in political strife and the peace of Europe to be continually disturbed by movements emanating from Belgrade."