So how much am I looking for a set of 19's and where is the best place to buy these from?
not sure where to buy from, for you. i live in australia, and there is an importer in this country for that brand, so it was quite easy for me. you would need to find the importer for the UK; perhaps contact the manufacturer.
they were good value, at around AUS$1000 a wheel. they are quite lightweight for this size (sub 11kg each) and very strong (central bracing rib runs along circumference of rim). the finish is really high; the wheel retailer commented these are some of the nicest quality he's seen of any wheel he has sold. price is excellent compared with original alfa 18s - they are considerably cheaper per wheel!
then you have to add tyres, for which i recommend 225/35. be mindful of the Load Index. alfa recommend at least '91' for the GT; this is easy to achieve with a higher profile tyre (ie: can cope with more load), and more difficult with a lower profile tyre. as such, my Toyo Proxes4 tyres have a LI of '88', which i considered acceptable. other tyres of similar quality include Pirelli PZero Nero, and my next set of tyres i plan on Goodyear F1 Asymmetric. Bridgestone RE050 are also available, but only in 'run flat' design from what i can tell, and i found the OE 225/45/17 versions to be 'hard' (stiff sidewalls), so i wouldn't buy them again cos of the lack of comfort.
be sure to upgrade the suspension too, especially dampers. this will minimise wheel damage (wheels less likely to 'drop' heavily into potholes), and will cope better with any increase in unsprung weight.