Most Forex brokers only offer trading of the major currency pairs -- USD/EUR, USD/AUD, etc.
What is the best way to trade more "exotic" currencies such as the Brazilian real and the Thai baht?
Use a currency ETF. there are many. Specific to your question there is
I don't happen to find a currency ETF for Thailand, so the closest you could come to a Thai currency fund would be something that's an Index fund ETF that is based on an index in the Thai Market such as:
Because that fund is investing in an index of stocks that trade on the Thai market, you are in effect investing in something denominated in Baht. This is spelled out in the prospectus where it discusses 'currency risk'. The problem is that you are however not investing in just the currency, but rather a broad index of stocks denominated in that currency. Still to the extent the market holds fairly steady, you get much the same effect of investing in just the currency. to the extent the market is moving, you get the net effect of what the thai market does, plus how the bhat trades relative to the dollar.
There are firms that let you do this. I believe that Saxo Bank is one such firm (note that I'm not endorsing the company at all, and have no experience with it)
Keep in mind that the reason that these currencies are "exotic" is because the markets for trading are small. Small markets are generally really bad for retail/non-professional investors.
(Also note: I'm not trying to insult Brazil or Thailand, which are major economies. In this context, I'm specifically concerned with currency trading volume.)
Keep in mind that not every currency is "tradeable", i.e.: convertible.
In fact, neither the Brazilian nor the Thai currencies are fully convertible, and the trading with them may be limited. There are 17 fully convertible currencies currently in the word, you can find the list here.