A Biostar M6TWL is based on the 810e chipset and does not have an AGP slot. As you already know, good PCI graphics cards are rare. You have these options :
1) A PCI card in the GeForce 4 MX class is occasionally offered at Baazee.
2) May be easier to find at eBay if you have someone to pay for it in the USA.
3) You could try putting up a Wanted ad in the Digit forum Bazaar section.
4) Even if you can find it, it's usually much more expensive than an equivalent AGP card. As an alternative, you could look for a socket-370 (P3/Celeron) motherboard with an AGP slot (e.g. an 815 motherboard).
The motherboard plus AGP card may be more expensive than a PCI card alone, but not very much more because the AGP card will be cheaper than its PCI counterpart, and you will have a much wider choice of cards.