I wanted to try my hand at grafting some Hybrid Tea roses onto a good rootstock, what type of rootstock would you suggest and where is it available?
Depends on where you live, climate and soil conditions.
Charlotte North Carolina - in the Piedmont area of south central North Carolina 
I’m guessing multiflora is your rootstock of choice there, although there are several others you could use.
Wisconsin Roses offers clean multiflora rootstocks.
Go to website. See bottom of page.