As a general rule don't buy from overseas websites unless it's for small value items (less than £15) otherwise you will get stung for import duties. That asside from any legal/brand issues. i once bought some shoes online from an american website as they 'seemed' cheaper but ended up paying nearly the same again when they went through customs!
Back to nails... unless you are qualified I don't think you will not be able to buy any professional brand acrylic products legitimately. There are various DIY sets available but I can't (and wouldn't!) advise on any of them.
If you are looking to do some training, you should check what brand your chosen training centre uses, whether they will supply you as a 'student' and whether you get any kit with your course. Hope that helps!