It's lovely to make your own Thai herbal compresses, and I think that for certain conditions it is necessary; but most of the time we turn to pre-made compresses made in Thailand, from dried herbs. Most pre-made compresses are warming formulas designed to work on achy stiff bodies. They generally have herbs in them that are beneficial for the skin, fascia, muscle, and sen layers of the body and have an overall tissue softening and circulation increasing affect.
Look for compresses that have, in addition to other ingredients, plai (cassumunar ginger), lemongrass, kaffir lime, calamus, turmeric and tamarind leaves. If one of these ingredients isn't in the formula you are contemplating that's fine, but for a good generic herb ball you want most of these, and you definitely want plai.
If you are shopping for Thai herbal compresses, whether online or in Thailand, I suggest beginning by getting two sample compresses. Open one of your samples up while it is still dry and take a look at the herbs inside. They should be crushed/broken, but not powdered.
You should see vibrant colors, and be able to clearly see that there is a variety of herbs inside. If what you see is washed out, hay colored, and/or singular herb uniform, they are likely not good. I have, over the years, gotten entire boxes of compresses that were essentially hay, so always check.
With the second compress, steam it up and give it a whirl. See how you like the smell, how long it retains its heat, and what the affect is on your body. Remember that often times compresses need substantial steaming before they are at their best. Generally I don't like to judge a compress until it has steamed for at least 90 minutes. I might start using it before then, but I'll save final assessment for after it has been in and out of the steamer a handful of times.
If you are buying compresses online and have a good source that you know ships to your country, excellent. But if you are feeling lost as to where shop, try looking on Alibaba. Despite being bigger than Amazon, Alibaba is a commerce site that many westerners have not become comfortable with. I recommend spending some time there as I've found some lovely compresses through it in years past. You can usually request sample compresses even if the seller has a high minimum order; but don't be surprised at having to pay for those samples.
Thai herbal compresses are incredibly therapeutic, and should be considered essential to any treatment oriented Thai massage practice. I always tell my students that it's best to go into sessions expecting to use compresses and have them steaming in advance. I do not suggest offering them as a separate menu item, as most clients will not realize just how wonderful they are, and won't choose to buy them as an add on to their massage. But also, because the compresses genuinely reduce suffering through alleviation of aches and pains, to offer them separately, as if they are a luxury spa item instead of an effective practical tool of your craft, is to possibly omit needed treatment from a session. Rather than charging clients extra individually, I would urge you to cover the cost of the compresses by raising your rates a bit across the board. And remember, so long as the compresses are not used directly on skin, you can use them for multiple sessions. Wrap the compress in a face towel, use through the client's clothes, and then re-steam. The compresses have a wall of cloth, are full of antibacterial herbs, and have the power of steam heat disinfecting them.