I asked on AIBU a while back, and some genius told me to buy converter clips on ebay and take my bras to a seamstress. Cost me £12 total to adjust one of my normal bras to a nursing one. It worked perfectly.
I always recommend converting. There are very limited number of underwired nursing bras in large sizes, and they are horribly expensive. (also careful when ordering the Anita Underwired - it has different sizing system and cups are smaller than you would think).
Converting is dead easy though and you can simply buy normal bras, way easier to find bargains as well. I even converted a lovely Fauve bra last time - much better support than any nursing bra will offer.