A few years ago Daniel Smith introduced these dot cards. At least I think they were the first. Since then they have proliferated. Winsor & Newton then offered a few, I think with some special edition sets and also a basic primary colour set with currently a six dot card from Jacksons at 20p. Schmincke now offer the whole range or - like Daniel Smith - a reduced number at a lower price. There may now be some other dot cards from other makers but I have no current details.
This is how Jane Blundell paints out the dot cards. I think her method is best.
Prior to this the options were and still are printed colour charts which are usually free or hand painted charts, often hard to find and quite expensive.
Prices of the dot cards vary. With Daniel Smiths huge range the full 238 set costs £21.00 at current jacksons prices. They also do a 66 one for £7.50. The full 140 colour Schmincke costs £14.60 and the 80 one £9.10.
Daler Rowney Hand Painted Chart
Holbein Hand Painted Chart
Maimeri Hand Painted Chart
I also have a Winsor & Newton hand painted chart. What do I think of them? They are undoubtedly useful, especially if you major on one brand. The snag I find is that the paints are applied very evenly. This may seem an odd thing to say but with my - admittedly - less than perfect methods getting a similar result is very difficult and do you actually want them to be this bland? Jacksons also list hand-painted charts from Rembrandt - no current price. Shin Han at £34.00 and - surprisingly - Cotman at £11.20. I believe Old Holland also do one but no details.
All makers tend to offer printed colour charts for free. My preference would be for these charts followed by the dot cards. You may feel differently.