If you're not going to be in HK for long I'd arrange your china visa before you go - (you dont want to waste precious HK time running around after your visa)
This is what you do
Download visa form from embassy website Fill it out
Book your travel and accommodation in Shanghai
Get a passport photogragh
Go to the embassy ( 118 Merrion Rd) between 9 & 12 Mon-Thur, queue up and submit all of the above with your passport (at least 6 months left to run on it and some empty pages)
Go back a week later (they'll tell you when to go back) queue a bit more and collect and pay for visa
(If you're travelling on business you'll need an invitation letter from whoever you are going to visit)
Simples!
About the suits - you can get these in 24 hours in HK or Shanghai but for really good quality suits you'll only save money on Savile Row prices not on ordinary high street prices.... IMHO
A Great market to visit in Shanghai for clothes / tailoring etc (they'll deliver to your hotel aswell) is Qipu Lu. 七浦路(近河南北路)(show taxi driver the addr in Chinese)
Enjoy HK & SH - stock up on your sleep before you go - both cities have an incredible energy!