OliviaMyee - try out these people Cypress Lodge :: Promoting Humane Training and Treatment of Horses
I buy all my lead ropes and halters from them and they are great. They tie rope halters to measure and they have a great range of rope leads and you could order them in any length you want I guess. Postage is pretty reasonable and quick - I normally get it within a week and its maybe $10 or so, they're based in Victoria.
These are the lead ropes: Cypress Lodge :: Promoting Humane Training and Treatment of Horses
I use the "Advanced" one which is slightly thinner than the rest - but most importantly it only has a loop at the top so you can loop it through any clip you like. They have leather flappers at the end which they will also stamp your horse's name into at no extra cost.
Really can't recommend these guys enough. Great service.