The front O2 on a 1998 SSEi and the 1999 SE (as listed by rockauto) should be an AFS109 (25312203). If you get the 5% discount, it should balance out, but here it is at another good AC Delco online seller:
I've used them about 5 times with never a problem. I use the Paypal option, but I think you can use a credit card direct. They have great customer service at 888-658-4675 X101. They cater to the B,F, and Y-body crowds, but stock or can get anything AC Delco.
If you go www.rockauto.com
(I've happily used them too), a current discount code is 650010587533 through 06/26/07.