The first round of documents were the letters of Paul to the churches, from 40-55ad...these circulated widely, and were available to anyone who could make copies...

The gospels were probably written up in the 50-60ad time frame (except John) and they somehow appear everywhere at once! (even John, written at Ephesus, shows up in North Africa within a couple of years!)...

These works were available to outsiders as we know from a couple of things:

1. The Gnostic 'outsiders' (as well as 'insiders') wrote about them often (see the Bias debates)
2. The first Christian apologists (100-150 AD) appealed to still-living witnesses of these things (e.g. Quadratus) and to fulfilled prophecy (e.g. Diognetus)...and DEPENDED on external access to this...
3. The first anti-Christian apologists (e.g. Celsus, 170ad writes) are VERY familiar with the NT writings...

All the data we have says that it was available early...

Also, you have to remember that these Christian communities before 90ad were parts of Judaism, and so, just as scribal documents would be copied and available from ELDERS, so too these early Christian dox would have fallen into that pattern...

if the dox WERE in finished form within 20 years of the Cross (and in oral and partial-written forms much earlier), and if they were developed within the Judaistic-synagogue environment (such as Acts described) then the people (especially the leadership which always followed Jesus around) would have had EASY access to it...remember Peter and Co. taught DAILY in the temple about these things at first...

also, a great number of priests had become Christians early on, and would have been interested in the written forms as well...