ECE Teacher here...
I'd like to throw in the suggestion to *not* limit yourself right off the bat to a particular philosophy or name.
You, for example, know why you like the Montessori programs you've seen, or what components of Maria Montessori's educational philosophy you like. This will allow you to observe programs looking for those things, even if they haven't paid for the name (and aren't charging you a premium for it). And believe it or not, there can be quite a few differences between Montessori schools as well--higher price doesn't always have to mean higher quality!
Visit (and have your child visit) any program that you've screened and decided that you'd like to explore further. You can find great preschools through your local schools, non-profit centers, in-home preschools and even (a very few) corporate child care programs. It all depends on the teachers and director of the program.
What are the things you're looking for in a program, and what do you and your family want to get out of it? Start with that list, and then evaluate ANY program very carefully based on that. There's no such thing as a too-invasive question to ask a program, and keep your eyes and ears open when you visit, don't get bogged down in educational lingo. And most importantly, trust your child's instinct. Straight Montessori isn't for every child. And almost every good program will include some concepts of it in their program.
My own personal preference is a mix of Montessori/Vygotsky/Reggio Emilia as a teacher, and it took me a long time to find a place that fit it (and was a non-profit center, too!), and when Fiona gets older, I may run my own preschool out of my home, since I love that age group (as well as all the neat activities you can do) so much. If I don't, then I will probably enroll Fiona in a part-time preschool program at the school I used to teach at (since I still maintain close contact with the teachers and director there).
It's important to me that she get some 'classroom experience' before she goes to kindergarten, so she's not weirded out from going from just me and her to 20 kids and a teacher. I plan on teaching her basic literacy and math at home, at her own pace, so my interest in preschool is less school-readiness and more social interaction and the opportunity to do REALLY cool art and science projects we don't have the equipment for at home, and a chance for her to make more buddies. And if it turns out she's just not ready for that until kindergarten, so be it--but I do want to give her the chance.

I hope that makes sense.