Re Nelson area, quite a few of the local cafes have exhibitions in them plus many artists open up their studios to the public and there are numerous leaflets available re various art & craft trails. Also many local craft fairs which are always asking for people to exhibit/sell. The craft market which someone else mentioned is fantastic but doesn't have much art of the painted/sketched variety, it's mainly things like wood turning, pottery, hand made jewellery etc plus loads of unusual things. Not to say that paintings wouldn't sell there but it's more of a high quality gift outlet, ie lots of tourists buy presents/souvenirs there who might not necessarily buy larger items such as paintings.