I'm worried about the same thing b/c my almost 5 year old niece still sucks her thumb. I will keep checking the responses to your post and see if anyone has any advice.
My daughter started sucking her thumb when she was three months old which was great b/c that really helped her stay asleep at night. I had tried getting her to take to a pacifier starting when she was 4-6 weeks old b/c I read that it might help to decrease SIDS. Well I kept trying and tyring but she never took to it. She is 7 1/2 months old and still sucks her thumb when she is bored, overstimulated, or sleepy.
We have several pacifiers around the house and she'll play with them--mostly biting/chewing on them b/c she's teething now. But she still doesn't suck on them except for a few seconds when she's playing with it.
You could try to give them to your baby to play with and she'll figure out how to suck on it eventually. I just don't know if she'll think it's better than her fingers...