I am not really of fan of this, but this looks like it could be made very easily. If I was more of a track rat, I would do this and put in a full cage. But thats not realistic on a daily driven.
http://cgi.ebay.com/ebaymotors/BMW-E...ht_1998wt_1167 This diffuser is 50$, plus 20$ for shipping.
http://cgi.ebay.com/ebaymotors/92-98...item5192693db7 I think this would be a simple instal. Cut out the outline of this, make some sort of mounting bracket behind the cover. Then with fiberglass, or bondo, you can fill in the space around it, or round of the edges making it look clean. You can have your bumper professionally painted, as well as having this painted in the black of your choice, and still be way under the 500$+ price of a ti sport bumper.
And Philly, these cars have been in around for 15 years now, I think they would have made a cheap version by now. I doubt we will see anything in the near future. If some cheap chinese knockoff company tried to make a cheap replica version it would be of poor quality. There is even a lesser chance that a high quality company would make something nice and still for a decent price.