I'm planning to breastfeed my first baby due next month. I decided that knowledge was my best support with feeding and so invested in two books:
The Food of Love - an entertaining illustrated book that's got great advice
The Womanly Art of Breastfeeding - a really comprehensive book produced by the LLL
I'm also going to go along to a local feeding class next week to see what support there is in my local area.
I haven't bought bottles, pump or sterilizing equipment. I don't think its something I'd want to do in the early days. If we change our minds then the shops are usually open and in an emergency we can improvise at home with hand expressing, a cup and sterlising with boiling water.
We also haven't bought formula, though we have discussed which one we'd use if we do need to supplement at somepoint. My feeling is that again we will be able to get some from a shop or relatives in an emergency, but that having it to hand would encourage us to use it when we'd otherwise persevere.
I've been given a partial box of breastpads that my sister had leftover, if I need them we'll get more. I've got myself a few cheap vests with narrow straps and I'm still to get a couple of nursing bras. I haven't made a decision about special pillows etc, I guess we'll just see what we need once baby is here - same goes for nipple shields, shells etc. I have got a tube of lanisoh.
I've got a little basket that I'm going to put things I might need whilst nursing when I'm back home - emergency snacks, muslin, a good book, tv remote, phone, breast-feeding book for trouble shooting, bottle of water etc. So that I can make wherever I find feeding baby comfortable into a little nuring station if my DH isn't around to fetch things for me.
Try to focus on what you are likely to need in the first week or so if everything goes well (breasts mainly!) and arm yourself with the knowledge of how feeding works and what common challenges can arise (and how to solve them!) so that you know what to look out for. A supportive partner/family and the numbers of local or national breastfeeding support groups are also helpful.
You can always pick up (order online or send someone out to get) any of the extras once you get an idea of what would be useful. That's what I'm planning anyway.