The single word that comes to mind after reading your description is 'serendipitous'. I won't suggest that word because it's already been suggested in the comments.
Other words seem less apt, but certainly there are many. It's true, also, that best would be a word fitting the individual occasion, rather than a general, all-purpose word (such as 'serendipitous'). So, considering the nature of your particular find, this word may be best overall:
providential, adj. and n.
1. a. Of, relating to, or ordained by divine providence.
["providential, adj. and n.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/153453?redirectedFrom=providential (accessed November 24, 2015).]
In your example, 'providential' works well:
I just made the most providential find!
Other good candidates, depending on what aspect of the find you want to emphasize, include
Of those, 'propitious' emphasizes the current circumstances at the time of the find:
- presenting favorable conditions; favorable ....
[propitious. (n.d.) Random House Kernerman Webster’s College Dictionary. (2010). Retrieved November 24 2015 from http://www.thefreedictionary.com/propitious ]
'Auspicious' looks forward to future circumstances:
Adj. 1. auspicious - auguring favorable circumstances and good luck ....
[auspicious. (n.d.) WordNet 3.0, Farlex clipart collection. (2003-2008). Retrieved November 24 2015 from http://www.thefreedictionary.com/auspicious .]
A usage note points up the distinction between 'auspicious' and 'propitious':
auspicious, propitious - Auspicious implies success in the future, while propitious means favorable conditions are present; auspicious means promising or of good omen—not just special or memorable.
[propitious. (n.d.) Farlex Trivia Dictionary. (2011). Retrieved November 24 2015 from http://www.thefreedictionary.com/propitious .]
Last, 'adventitious' emphasizes the accidental, unexpected nature of your find:
- added or appearing accidentally or unexpectedly.
[adventitious. (n.d.) Collins English Dictionary – Complete and Unabridged. (1991, 1994, 1998, 2000, 2003). Retrieved November 24 2015 from http://www.thefreedictionary.com/adventitious .]