I shall be making a trip to Zara today to see if I can find their recommended jodhpurs, but when I first started with the obsession I made my own from a pair of old green trousers from a charity shop. The problem with these was the fact the trousers weren't baggy enough to begin with so the effect wasn't dramatic enough for me. Then Queen Marie bought me some amazing Belstaff ones which were properly baggy at the hip area, but they are made of very light cotton, so won't hold up for winter, hence the reason I need some heavier duty ones.
However the principle of converting basic, loose trousers with simple seems (i.e. no double stitched seams like you find on jeans) is extremely easy.
Imagine cutting a shape of a narrow champagne flute - that's basically the jodhpur pattern (well, it's how I do it anyway. No doubt there is a proper way).
The red dotted line is the shape you will sew, and the black dotted line is where your knees are. I advise choosing trousers that have a slight stretch and are a good weight for maximum exaggeration.
When you sew the red shape it's important, in my opinion, to get as smooth and gentle a curve as possible which smoothly joins the existing seam at the top part of the trousers.
And that is how you make a simple pair of jodhpurs. Although, I have it on very good authority that Topshop are working on a design as we speak...so you might want to hand on in there and save your thread.