I have an old Nikkor-S pre-AI 50mm f1.4 lens that came with the Nikkormat my dad gave me. I found someone in the states on ebay who did a great job of it for me, at very reasonable price. He only actually needed the aperture ring part from the back of the lens, but I wasn't able to remove it myself so sent him the whole lens. He also kindly cleaned out a little bit of fungus that was in it at the same time. While the Ai'ing doesn't look quite as good as a proper AI job with the correct kit, it was done to a high standard and I was very happy. I'll try to find him again on ebay and update. (update: doesn't seem to be there any more as far as I can see).
Also worth finding out which conversion kit your lens needs and see if you can find it on ebay or elsewhere, because that will be the best solution by far.