A makeup kit is not cheap and it takes a long time and loads of money to reflect your client base.
Most companies (apart from the designer ones) and makeup stores, will have a professional makeup artist scheme that you could apply for; proving that you work in the industry (by showing various items like):
copies of your certificates
a business card
a call sheet to show that you have been hired for a job
union card with photo id
any published or credited work
Some of the companies that offer the scheme are:
MAC - provides 35 % but you have an annual fee of approx £26 to pay
Illamasqua - provides 40% discount no fee to pay
Bobbi Brown - provides 40 % but you have to pay an annual fee again
Makeup Up Forever
Stores - PAM and Guru Emporium
You should apply to the each company either via e-mail or by post.
Please add to this list if you know of others that provide a discount.