Many terrestrial orchids can be difficult to cultivate, while others are very easy. It really depends on what you're looking at.
However...
From what I understand, Thelymitras are not necessarily the easiest terrestrial orchids to grow. The reason being is that they don't really produce a whole lot of roots per plant (probably less than 5 roots per plant). If you damage one root, it'll be hurtin'. I've also read that the underground tuberoids can also be prone to rot, particularly if they're kept either too warm, or if they're grown too wet during dormancy. Then there's the issue of how dependent each species of Thelymitra is on their mycorrhizal fungi.
If I'm not mistaken, dormancy is during the summer. Active growth occurs in the winters. But do remember our summer is Australia's winter.
Unfortunately, if you have any further questions regarding cultural needs and such, I cannot answer many of your questions as I don't have personal firsthand experience with this group of orchids myself.
And yes, there is some interest to form a group order for Australian native terrestrials. From what I know, they will all be from cultivated or tissue cultured stock.
There should be a list of Thelymitras available in Nov. There will most likely be both species and hybrids on the availability list, I'm not 100% sure quite yet.
Thelymitra formosa will most likely
not be on the list.
Thelymitra nuda or Thelymitra pauciflora may be, again idk yet.
They will also
not be from the US - be aware of this. There are costs that will be incurred aside from the cost of the plants themselves. If you are serious about Thelymitras, you may send me a private message and ask about the costs.
As an fyi - the costs I'm referring to will go into CITES and phyto permits, as well as shipping and handling costs. I don't make any money off of these costs, btw. I can offer proof of what the actual costs are once the list of availability arrives. They
will not be cheap, hence the initiative for a group order. If there are any complaints about these extra costs, please take it up with the Australian government.
From what I've gathered no one in the US sells Thelymitra spp.