Hi! I'm from the UK, and I use contacts and glasses so what you want to do it:
If you're under either 18 or 16, can't remember which it is, but optical express do free basic eye exams. You can go, have your eyes checked over for what you need to know to get contacts.
Don't know how much it is if you're over 18, my eyes haven't changed since I was 15 so I've not been back.
Anyway, with buying the actual contacts, if you go into costume shops, and some beauty shops (Not boots/superdrug) they stock a brand called 'mesereyes'.
They're okay, they don't cover too well, but if you can't order online or just need them in a rush, they are okay. They do fashion lenses and then the basic colour halloween ones. The base curve for those is 8.6 with a diameter of (I think) 14.00, I have never used them, because I need my contacts to have a power or I'll be walking into things all through the con, but my friends have used them.
But for Lucy, because her eyes are such a bright colour, I'd use some from pinky paradise. You can order them with or without a power, then to me, in nottingham, it took 6 days to deliver from malaysia with the cheapest shipping.
the contacts last for a year, and you can get those to make your eyes look really big.
The mesereyes contacts cost about £10-£20 for a pair that can last 3 months, tops, where PP's are about £20 inclu shipping, but last for a year so you get so much wear out of them.
But for lucy, my recommendation is: