Does anyone know of a 'garden centre' in N Thailand that sells  rose plants either floribunda or HT hihc ar correctly budded onto a root-stock, not rooted cuttings which is all I have seen in Thailand?

North Thailand is too wide an area . I can only speak for Chiangmai and around.

Kam Thiam behind Tesco Lotus in Pa Tan or Buffalo Market on the south side of Sanpatong on a Saturday.

The area Maybole suggests behind Tesco Lotus located on the “Super Highway” in Chiang Mai has many outlets, whether they meet your needs re grafted etc is another matter.

The other issue is identifying the type, I managed to locate some I used to grow in Australia such “Double Delight” only because I know the type.

Don’t know where you are from but they can be prone to black spot due to the humidity something we used to have to deal with in Sydney.

Plenty of stuff here to treat them with

I've searched high and low and can only find rooted cuttings. Perhaps you could try grafting a cutting from one variety to the rooted cutting of another variety to achieve what you require.

It's true that although cuttings will grow, sometimes very well, for a few years, they do eventually succumb to the ravages of soil borne disease. 

The amount of work that was done in order to 1: develop a rootstock that worked in temperate climates and 2: to propagate and get into general use was quite impressive. I don't think that Thailand is a suitable place for the traditional rose and so can't imagine a 'tropical root stock' being developed here. 

Maybe in Singapore or some other place where the English were established for a few years?

My wife buys roses from the North of Thailand almost on a weekly basis to get them boxed and delivered here in Phuket: she orders them all through Facebook on some Thai pages. You can check out these 2 businesses and see if they have anything you like:

https://www.facebook.com/Rose-in-love-Junior-113041676804807

https://www.facebook.com/FamilyRoseGarden

There is a huge Rose Garden/specialist/farm/sales outlet three km’s east of Samoeng on the famous Samoeng loop!

you will pass it if you come to Samoeng from ChiangMai whether you come bt Mae Rim or Hang Dong route!!

4 hours ago, Brad Grier said:

There is a huge Rose Garden/specialist/farm/sales outlet three km’s east of Samoeng on the famous Samoeng loop!

you will pass it if you come to Samoeng from ChiangMai whether you come bt Mae Rim or Hang Dong route!!