Today there are over 180 extant Coptic MSS of the Gospel of John in the Sahidic Coptic dialect, as reported by Karlheinz Schüssler in his overview, “Some Pecularities of the Coptic (Sahidic) Translations of the Gospel of John,” Journal of Coptic Studies 10 (2008): 41-62.
Most of these MSS are fragmentary, but there are five complete MSS of John (i.e., sa 505, 506, 508, 561, 600), thirty-eight lectionaries, and three other liturgical MSS. Schüssler has published facsmile editions and transcriptions of sa 505 and sa 506 online on his website Biblica Coptica.
Both MSS are from the Monastery of Apa Jeremias (Saqqara) dated to around 600 C.E. These editions contain images facing transcriptions presented in a convenient e-book format.
Karlheinz Schüssler is editor of Journal of Coptic Studies, and Biblia Coptica, a series which lists all the Coptic biblical mss. He is currently working on a text of John in Sahidic for the International Greek New Testament Project.
More on Schüssler's personal webpage.
For Coptic resources on-line, see here (Coptica) and here (compiled by co-blogger Askeland).