From my understanding, you can make your shahada without witnesses, although it is recommended to have witnesses if possible. However, if that is difficult to organize, you can say it without witnesses. I thought you had already done so, actually, but maybe you said it just to practice without the intention of entering Islam. Anyway, you shouldn't delay taking shahada just because you can't find witnesses.
I'll tell you what happened in my case. I made shahada without witnesses and began praying etc. The next day I went to an Islamic centre and asked if I had to do anything else to make it 'official'. They said no, but that if I wanted I could say shahada again at Friday prayers. A sister (who ended up becoming a really good friend) helped me and took me to the masjid. At a certain point the imam asked us to come forward and I repeated the shahada after him (from behind the hijab - no need to leave the women's section).Later that day they gave me a kind of document confirming I had reverted, but to be honest I think you really would only need that if you were going for hajj - it's not essential. The important thing is to take your shahada as soon as you are sure. If you feel nervous in front of so many people but want witnesses you can say it in front of two Muslims.
If you want to say it in front of witnesses you should try to find an Islamic centre where they can help you. If there isn't one, I'd say just go to the masjid find a sister and just tell her your situation. Or get in touch with that sister you met and ask her to help you. But don't worry about it and don't delay if you are sure.
You already know how to pronounce the shahada and the meaning, right? If not we can help you insha Allah.
I'm only speaking from my own experience so I hope someone else will correct me if necessary. All the best.