Does anyone know where to get tefillin parts, specifically retzuot and batim as a friend of mine needs new batim and I need new retzuot.
If you need to order online, you could try Oter Yisrael - even though they don't seem to have those items for sale, they are very friendly and may ship you individual items. (Personal recommendation; I've bought many pairs of Tefilin from them over the past 30 years for my brothers and sons.)
Another place which has an online presence with a very good reputation is Tefilin Bet El, though I've never actually been there. They seem to sell Tefillin straps online.
If you are in Geula there's a great place just off Kikar Shabbat at the top of Yechezkel where I replaced my Tefillin straps recently.
Or you could go to HaSofer - or visit them virtually. They sell straps online.
The only ones selling Batim online seem to be GooDaica - but I know nothing about them.
http://www.judaism.com/giftware/tefillin.asp?nt=Bm indicates that they sell tefillin entire, but e-mail them: they will sell tefillin parts also. The address is firstname.lastname@example.org.