Easiest are most of the Sri-Lanka species: C. beckettii, wendtii, walkeri, undulata, etc. C. walkeri is usually sold as C. lutea.
also pretty easy are: C. crispatula, var. balansae and several other Crispatula varieties; C. pontederiifolia, C. cordata
Some people, but not everybody have difficulties with, C. aponogetifolia, C. lingua, C. usteriana, C. parva, C. affinis.
The rest are hard to get. Most of them are also hard to grow, but not all.