First, about your supply, there are medicines/herbs that help you produce more! Also, if you can pump in between feedings, this should help!
Second, introducing cereal/solids, most pediatricians say to wait until 6 months, but I started my daughter on cereal and solids at 3 and 4 months! Mommy knows best! If your daughter follows your spoon/fork to your mouth, she is ready, if not, try pumping and adding a small amount of cereal to the bottle! bottles work for baby food too! You just have to be sure the slit in the nipple is big enough for her to suck the food out, but not too big that she chokes!
Third and final, to wean w/o your supply leaving too, well, I am not too sure on this one, as I nursed until my daughter was a year and then put her on a sippy cup! She had sippy cups from 6 months and up, so she was used to them! I hope you get better advise on this last one!
Good Luck, and I hope you figure out what is right for you and your daughter!