Logee's has citrumelo. not sure if it's Dunstan or Swingle but the catalog description says it tastes sort of like a sour grapefruit.
Be warned though, the size is very small so you will probably need to grow it a year before you can even think about grafting.
Anyway, if you just want to practice grafting you can easily practice grafting onto an ordinary citrus tree from the nursery. For example, tangerine can be grafted onto an orange that is itself already grafted onto dwarf rootstock.
