Tefillin
Tefillin are small black leather boxes containing parchment scrolls inscribed with verses from the Torah. A set of Tefillin includes one for the head and one for the arm.
Stam.net has an excellent and detailed description of the function and manufacture of tefillin. The Wikipedia entry on Tefillin is also extensive.
I also recommend the book Tefillin and Mezuzos: A Pictorial Guide by Yerachmiel Askotzky. In this book “A sofer shows you how to choose, maintain and understand your tefillin, mezuzah and Torah scrolls”.
Buying Tefillin
Tefillin are tricky to make, so they are not cheap. The housings are complicated, and the writing on the parchments inside is extremely small and subject to an awful lot of rules. Thus, if someone is offering very cheap tefillin, there is something wrong somewhere – either corners are being cut or someone is being exploited. Either way, not good.
I make complete sets of tefillin or contact me if you just want parchments.
Buying tefillin from a soferet
Tefillin, like other sacred scrolls, are traditionally held to be invalid if written by women. If you are non-egalitarian, even if you are a woman, tefillin written by a woman cannot be kosher for you, no matter the circumstances. Therefore, even if you are the most feminist of feminist Orthodox women, you should not acquire tefillin written by a soferet. Sorry; them’s the breaks.
If you are very sure that you reside in the segment of the Jewish world which in principle extends eligibility to write tefillin to women, then you can consider getting tefillin from a soferet. If you do that, please remember not to lend your tefillin to non-egal Jews (men or women), because to them the tefillin will be pasul and it will be like feeding them non-kosher food – not a nice thing to do at all.
Borrowing tefillin
If you want to take on the mitzvah of tefillin but can’t afford them, or if you want to try it for a while in order to decide whether you want to invest in a pair of your own, it is possible to borrow a set.