There are many varieties of aubergines/eggplants that are used in Thai cuisine; the most common ones are Thai green aubergines (about the size of a golf ball) and the pea aubergines (about the size of grapes). They are quite bitter and are used most commonly in green and red curries. You need to cook long enough until they’re tender and lose a little of that bitterness.
In my years of cooking overseas, I’ve always had to use alternative ingredients when I can’t find Thai ones. Although many Oriental supermarkets now stock these aubergines, they’ve travelled a long way from South East Asia and aren’t always very fresh. If you can find fresh ones, freeze them! They’ll last a couple of weeks. If not, I’d recommend using purple baby aubergines. It’s not as bitter and the colour might look slightly different from the green or pea aubergines when cooked, it tastes great in Thai food and the texture is similar which makes it a great substitute!