If you can get old Nicad packs, then you can use them as far as I'm aware (haven't read the rules recently) but yes they are no longer allowed to be sold commercially. There are packs that come up for sale now and again on here but they're old packs that will have been used. Yes they might be fine but they'll have seen battle in most cases.

Nimhs are the next progression. Similar in size & stats but I don't think they are/were as good at providing as high a discharge current as Nicads. For the money, you'd be as well using Lipos. If you're unsure, just ask the right people lots of questions on them, read all the rules regarding them, and have a look at how other HW users have fitted them. If you follow the right procedures and treat them properly, they'll just be like any other battery.