You don't state, but it sounds like you have an automatic transmission.
If so, you may want to look into Amsoil, Royal Purple, or Redline
synthetics to see whether they have something comparable to the "very
specialized BMW fluid."

If you have a manual tranny, then you can use any of the above
synthetic oil and cost should be about $6-10 per quart or about $20
max.  Just change every 30K miles or so.

Also, don't forget to change your differential fluid while you're under
there.

Good Luck!

WARNING:
There is a strong possibility that a flush will kill an UNserviced tranny...
if auto.
If serviced every 50k miles, no problem, but if you have 150k and it has
never been touched, be careful what you do to it.

You should not flush it, only change the filter and the fluid that comes out
with it.
read more about this here (search for it, very common topic)
http://www.bimmer.info/forum/forumdisplay.php?f=10&page=1&sort=lastpost&order=&p
p=60&daysprune=-1

It depends which gearbox it is as to which special fluid is needed and
whether it can be bought elsewhere.  If you do a lot of googling you'll
probably find the info you need.   Personally I use www.e38.org 'cos I
have an e38 but much of the stuff there may also apply to yours.