Rolling for Tamamo?

I think You should wait, she will be available on Halloween

the odds are pretty low to get Tamamo on the story banner, I advise you to wait for a rate up banner.

If tomorrow, they are going to annonce the next chapter coming next week, we may have a rate up for Tamamo (along others 5* stars) on the London Banner (They do usually a banner for each singularity with their featured servants before a new chapter). It is still better than the story banner.

As an Tamamo-fan as well, I can feel your suffer T__T. I wasted like 900 quartz in total at Halloween, but gor Orion and Jeanne. After saving for 5 mounts I got her only at Nero Bride gacha.
So I was making a little research about her rateupts and... well... she's got only one clean banner per year - at Halloween. It is a best opportunity to roll her, so I highly recommend you to not waste quartz at story gacha (even it has a pic of her) and save as much as you can for october. It will be difficult, but it's the best way.