Without a doubt the best resource for Charles Rennie Mackintosh is the books by Roger Billicliffe, an acknowledged expert on the subject. In particular one of his books "Charles Rennie Mackintosh - The Complete Furniture, Furniture Drawings & Interior Designs" (publ. John Murray Publications Ltd, 1986, ISBN 0-7195-4318-5) will probably have what you require, a set of drawings of the
Hill House Chair, but it is now long out of print and currently fetching £150 to £200 secondhand (ouch!), so you may have to get the local library to request a copy on loan from the British Library. There is a more resonably priced book by Mr Billicliffe, "Mackintosh Furniture" (publ. Cameron & Hollis, 2000, ISBN 0-906506-01-8 ) which has photographs of the tall chairs and other furniture and may serve as an inspiration, but personally I know of no other source of drawings I'm afraid. When I made a Mackintosh-inspired chair for a client a few years back I worked from photos to get the proportions then scaled to get the correct size, made-up rods and templates and worked from them - so no drawings, I'm afraid, although the actual design is relatively simple because the back is absolutely vertical. I recall that steam bending the 30 or so back rails, plus the top back panel (26 or so pieces including the 6 bent rails) and cutting the 80 or so mortises required to make the chair was extremely time consuming, even with a square chisel mortiser.
I know that Woodsmith over at A T T (Ask The Trades) has also made one as well and may have even published plans in the Woodworker. A photo of his MDF Mackintosh-style wardrobes with a 2/3 size ladderback chair is on
UK Workshop.
If you want to get a feel for Rennie Mackinstosh stuff there's a number of places to visit - the Basset-Lowke (he of model engineering fame) house in Northampton at
78, Derngate and
Hill House in Helensburgh, Scotland amongst others. You might also want to try the
Charles Rennie Mackintosh Society to see if they can assist. Their web site includes some photographs of his furniture (see His Life & Work / Collection / Furniture).
Good luck!
Scrit