Don't buy any luxury goods in Thailand, the luxury goods tax rate is such a rip-off. But if you have to buy it in the new airport duty free shops are rather good, the shops are so much more impressive than the Heathrow or JFK (there was every brand you can think of, even Cartier has a huge shop there). But if you really want to go to the shopping malls, 2 with all the luxury goods are called Emporium and Siam Paragon
Other than that, I don't really know. Chatuchak is probably the diverse place to go because you can buy not only fish but (illegal) wild animals! Mah Bun Krong as seahorse suggested is wild with lots of fakes if you like that, LOL. Anyway it's opposite the road from the more upscale Siam Paragon so there is no harm to peek there.
And don't ever get on the 'tuk tuks' they rip you off big time and for the normal taxis always insist on them turning on the meter otherwise get out! How do I know? Because I can speak Thai (dad was in the S-E Asian diplomatic circles yonks ago so I am vaguely familiar with local languages) and I have heard the drivers of both 'tuk tuks' and taxis speaking about ripping foreigners off!
What to buy from Thailand? Naturally silk - Jim Thompson (this guy was an ex-CIA agent setting up his silk workshop in the 1950s if I recall correctly) is standard. Actually it depends on which type of silk do you like. Thai silk is a bit heavier than Italian or French silk just out of the different species of silk worm. There are other things to buy but I don't know what you like and other ladies can probably advise you better!