Consignment in Wellington: On Course Consignment shop. 10 minutes from the showgrounds. You can find it on the web. Lots of stuff; tack, clothing, etc. Happy Hunting! New tack can be had at the Tackeria, 1 minute from the showgrounds.
There are also tack shops that set up on the show grounds in Wellington during WEF, although I don't know if they will be there yet. If they are, Beval in particular usually has quite a few used saddles for sale.