Not just tefillin made of gewil (i.e. גויל - unsplit hide), but with the shin that goes all the way to the bottom of the bayith?
Is there any source for purchasing such tefillin?
Thanks for your inquiry about obtaining tefillin according to the Yemenite understanding of the Rambam.
First, you should know that according to the Mishneh Torah, tefillin are NOT written on gewil, but on galaf (i.e. קלף - parchment) [see Hilkhoth Tefillin 1:3,8,11]. Not only is writing them on gewil not the halakhah, it would not be physically possible or practical since it is so thick that your tefillin would have to be much larger than even the largest tefillin produced today.
As for where to purchase them, as these tefillin (i.e. with the baseless "shin" extending from the base) are so rare, I would suggest that you get it touch with either Rav Ratzon Arussi, Rav Eliyakim Tzadok, or Rav Mosheh Sarum through Makhon Mosheh (www.net-sah.org).
If you cannot find a pair like this, know that such a shin (or any at all) is not me`akev to their kashruth and purchasing a set through any Yemenite/Rambami source would be fine halakhically.
Hope this helps.
Good luck and Kol Tuv.
This answer serves as a comprehensive guide where one can purchase Rambam tefillin. If ever this information is outdated please comment and i'll update it.
In my travels to Jerusalem i met a sofer who makes special limited sets of Rambam tefillin in various sizes. His name is Steve Bar Yakov Gindi and I've been to his workshop and seen the quality of the batim he works with. He has a digital storefront but it doesn't list all the options available. The store only mentions tiny sized Rambam tefillin but you can request medium and large sized.
There is a Temani store called Nusach Teiman that sells tefillin at a very reasonable price, but I haven't seen the product with my own eyes. http://www.nosachteiman.co.il/?CategoryID=1041&ArticleID=5118
Another source is Machon Gewil, sometimes their website works, sometimes it doesn't. Here is the link:
Another sourse is Torath Moshe, note that there seems to be some overlap between them and Machon Gewil. I have spoken to the owner of this website and he was kind enough to tell me that he used to source the tefillin from the Steve the sofer who I mentioned above .Also, their website is unfortunately often down as well:
This answer will be only according to Rambam since that is what you asked. Tefillin may not be written on gewil and are passul if they are. (Hilchot tefillin 1:9) You can find tefillin written on Rambami klaf, according to the (probably) more accurate girsa about what klaf actually is, from Machon Gewil
You can purchase Temani (Yemenite) tefillin here. It is the only place I know of that sells them. They also sell other Temani things in their store, including mezuzot, Sifrei Torah and Megillot.
I must note they mention specifically the soferim writing on authentic klaf and gevil.