I treated myself to one of those U-shaped pillows, and it was invaluable for supporting my arms while feeding, and even providing support for the baby (to stop her rolling away). 14 years later and we still have the thing, but with a replacement cover - I use it for reading in bed now.
I also got in advance: three nursing bras and breast pads, Should have got Lansinoh from the start, made the mistake of waiting until my nipples were really sore before using it.
After about five weeks, once BF was really established, I got a pump, 6 small bottles, and a set of bags for freezing milk (they have a measurement scale on the side so you can know how much you've expressed, and you write the date on the side when it was pumped/frozen). DD1 tended to be asleep around 11pm, so i got into the habit of pumping then every evening and freezing it. When dd1 was 6 weeks I started very limited work - away from home for just 5 hours - and DH used the frozen milk with the bottles. We never bothered with sterilizers, but boiled the bottles for 10 mins each time.
To start with I rented a medical grade pump - huge brute it was - from a chemist. Once my body got more used to the process I bought a cheaper one.
One thing I never did was get formula in 'just in case'. Like Yellow said, it's setting yourself up to fail. If ever you feel there isn't enough milk, the first solution is to keep on and on feeding, which will increase the supply. If that doesn't work, it might be tongue tie or some other issue that needs investigating, but nothing that needs immediate formula feeding.