Offal need to be very fresh - especially kidneys, which will quickly taint and become bitter. It should look glistening and wet (not slimy) and stand firm and proud. Dry, cracked or bad-smelling offal should be avoided.
Liver and kidneys are probably the most widely available offal and are as simple to cook as a steak, but far less expensive. For a milder tasting offal, look for heart, sweetbreads or tripe.
It isn't always easy to find brains, hearts, trotters and tails, either, as supermarkets rarely sell the more challenging cuts. Making friends with your butcher is probably your best bet for securing fresh offal and butchers serving an ethnic community are a particularly good bet. Also ask at farm shops or small producers (you may need to order in advance).