I am curious as to why you seem so certain that she got the Salmonella from her formula. Did the doctors test it and tell you that?
Salmonella can be transferred easily by unwashed hands and cross-contamination (one surface to another)... ie, the grocercy cart that has leaked from a package of raw chicken gets touched by your purse (or your hands pushing the cart), which you touch, and when you pick up your baby or handle her pacifier... voila. That sort of thing. Or her pacifier gets dropped on the floor and rinsed for just a moment before being given back to her - but not washed thoroughly enough to really remove all the germs it might have come in contact with.
Also, it is VERY common with reptiles. Do you have a reptile in the house? I see you have a young stepson. Does he handle frogs and such outside? Salmonella can be acquired directly from animals such as pets, birds, fish, dogs, cats and turtles. Do you monitor him when comes from outside into the house to ensure he washes with soap before he touches anything else (doorknobs, too, not just toys)? Does he use the bathroom on his own? If so, does he wash thoroughly with soap? (Salmonella is present in the human intestine as well).
I am not pointing blame anywhere, just reminding that is is NOT that unusual to find salmonella lurking around your home. Salmonella is one of the most common food borne illnesses.
You didn't mention whether you have pets in the home or not, but if so, that could up the odds of her getting it.
Some folks (particularly grandparents) are very lax about germs and babies and think we newer parents are too germ conscious.... but there are reasons we sanitize where our tots sit in high chairs at restaurants, and grocery cart handles, and fret about whether or not a visitor has washed their hands before holding the baby, etc.
Assuming that you haven't blatantly disregarded "common sense" about your baby... it is still quite easy to see how something cross-contaminated could make it's way to your baby accidentally. Please don't blame yourself.
Yes, it could be the luck of the draw.