There is a fairly large ferriteria shop in P. Santiago, in the street opposite the post office. It looks smal from the outside, but actually goes back a long way. Can't quote you prices, but when we were there they were cheaper than Chafiras on many, but not all things.
The other possibility is Leroy Merlin, but they're at La Orotava & La Laguna. They seem very cheap on some items.
www.leroymerlin.es Hope you manage to get it sorted OK
Ed