I had a couple of (mostly worn out) nursing bras from my mom. They didn't fit well at all but they did get me through the first couple of weeks until I was past engorgement and able to get a good idea of *my* size and order proper bras. I'd say it's not a bad idea to get a couple of cheap (or even used) nursing bras for that purpose. I would not have wanted to get $40 bras that only fit for two weeks, you know?
That said, I am larger busted, and having some top quality nursing bras is VITAL. If you are small, you may be able to get away with tanks. I can use tanks now, but I couldn't for about the first 10 months. I didn't pay for nursing tanks, i just got the ones with a shelf bra and pulled them down...of course, I didn't wear them alone either--I wore them under my regular shirt. That way they covered my belly--tee up + tank down = modest nursing.
I had to wear a bra night and day for the first 10 months because i leaked so much even at night and I needed the bra to hold the pads in place. Seriously, the right bras make a world of difference.
I have had a couple of nursing tops and honestly they drive me crazy. I have gotten rid of all but one because the layers and slits are just a hassle. Wear plain shirts that are a little loose or a little stretchy, and just pull them up. (In other words, wear your normal clothes!) I had a couple of tops that were not nursing friendly, but most of them adapted just fine.
BTW, I got my nursing bras at www.birthandbaby.com
and I was REALLY happy with them. They are super helpful if you call the number and they can help you find what you need. I thought I knew what I wanted, but also got the bra they recommended to me--turns out I like their recommendation better than the one I picked out myself! Go figure!