Does anyone know of a company that sells, delivers and plants semi mature trees in the Chiang Mai area?

I would be very careful about this if I were you. I recommend that you start with younger trees. Many species grow fast in tropical conditions and out-grow the more mature plantings that have been compromised by poor transplanting methods used for larger trees. Patience and cultivation of good growing conditions is to be considered.

 

Local practice for digging, boxing and moving mature trees is very extreme and does not provide an adequate soil/rootball.  Best Management Practices established by the International Society of Arboriculture for planting and transplanting are rarely observed. The severely cut roots in some species get infected with soil borne pathogens like Phytophthora spp, that in time progress up the roots and into root collar and create lower trunk decay and structural defects.  I have seen many "semi-mature" and mature tree plantings in Chiang Mai area that appeared to do well for a few years, but then deteriorated as the pathogens advanced and caused physiological and structural problems, with potential for decline in tree health and risk of structural failure, property damage and personal injury. 

As above, we planted small trees and also grew many from seed.  18 months on and they were large trees, now at 3 years a (from seed) poinciana is 12m, black afara's @ 9m and bauhina planted 18 months ago are @ 6m.

 

Kampthieng Market have a wide variety of semi-mature trees and seedlings.  In our experience the youngest trees, say 50cm tall, grew faster than more mature 2m tall specimens.

I went to the Kampieng plant market and bought a Cassia javanica tree, about 1m tall with a nice root ball. This is an auspicious pink blooming Thai tree and should be in every yard.  It's a real beauty. 

I only paid 150 baht for this life long enjoyment.  Now after 2 months in the ground it is showing good signs of stem growth so its future seems assured.

More here:  http://toptropicals.com/html/toptropicals/articles/trees/cassia_pink.htm

I hope I'm not too far off topic but can a walnut tree be purchased here and would it grow?