If you want something used and cheap, and are willing to play "look all
of them up including FCC ID numbers first", then the "Computer Swap &
Shop" up on Johnson Drive has a shelf (with nifty wooden vertical racks)
of used NICs, ISA and PCI, which they are letting go for $2.50 each (the
shelf is their "1/2 Price" shelf and most cards are priced at $5 on their
sticker).  They also have modems of various speeds, a 33.6 goes for $5
(again, its marked $10 but its on the "1/2 Price" shelf).  A 14.4 you
could still use to send and receive FAXes is $2.50, and I think all of
the 14.4s I've seen there are hardware modems.

Computer Swap & Shop is at 5612 Johnson Drive, between Lamar and Nall on
Johnson.

If you want something new with a shrink-wrapped box, my personal favorite
for all my Linux systems (I have three at home) has been the "Hawking"
10/100 NIC for $9.99.  I've never had a Hawking fail on me, and I've
installed them on other people's computers as well.  They come with Linux
drivers, though Mandrake has never failed to notice them and install them
using some driver which comes with Mandrake.  You can find the Hawking
10/100 NIC at MicroCenter at around 93rd & Metcalf, in the same shopping
center as Borders on Metcalf.  These days when I need a new NIC,
especially for a Linux box, I buy a Hawking.  Plain and simple.

Incidentally, you'll quickly notice if you start recieving MicroCenter
ads that they *never* put the Hawking NIC on sale.  It stays at $9.99 all
the time.  There are days I come in and the rack next to the Hawkings is
stuffed with "Currently marked $9.99 NIC, but FREE after $5 instant
savings and $5 mail-in rebate" NICs, and the Hawking rack is EMPTY.
Seems other people are having the same good experience with Hawkings as I
do.

(yes, I admit it, I go into MicroCenter frequently, partly to gaze
longingly at stuff and partly to be so near so many computer parts.
However, I buy my blank CDs at SAMs Club, $22 for a spindle of 100
Verbatim blank 80min/700MB CDs.)