I look for cheap nursing bras at thrift stores and snatch those up, (pay no mind to the quality or the size of them- it’s the hardware that you want from them). I cut out the clips and straps from them and sew them into my bras that already fit me well.
Please keep in mind, I wear wire bras. I know that the trick with a wire bra is to have a proper fit. The wire should not dig into any of your breast tissue – meaning the wire should lay on your rib cage, just outside of your breast. When you’ve got full breasts wearing a wireless bra is not always an option – they just don’t support the girls the same way a wire does. Please remember, if you do start wearing a wire nursing bra to do it *after* your engorgement stage.
Not sure on your fit to begin with – especially while nursing? Then check out my post on finding the right fit for you: You Might Be Wearing The Wrong Size Bra.
Here’s some photo examples of one of my (finished) converted bras:
My homemade nursing bra. Click on the image to view it larger.
You can see how I hand stitched the claps in. Click on the image to view it larger.
The Joy of This 