Looking at the pictures of youre bike i cant tell what the chain size is. However it looks like a standard rear sprocket so 630 i'd guess.
Easy enough to tell, just a rough count of the teeth and there it is. Around 98 to 102 its 630. 112 to 114 ish and its going to be 530 probably.
If you want to stick with 630 save youre money i have several rear 630 sprockets you could have foc. I have a decent front too i believe though none are new.
You can get chain and sprockets from several sources and in several grades for the chain.
Debbens, M&P, Busters, Talon, Wemoto, etc etc etc.
http://www.mandp.co.uk/productinfo.aspx ... -REGINA-CZ
This is one example of from M&P but dont know the quality or size but i suspect 630 and standard sprockets albeit an economy chain.
£71.59
If you want custom size sprockets i would suggest Talon and they do keep some on the shelf. I bought a rear non standard from them a month ago and it was in stock.
They also do off-set fronts if thats the way you want to go.
Rear tyre size from another thread in discussion should be 120 section but i have a 130 on a gpz rear rim like yours and its fine. No need to alter sprocket off-set with this size of tyre but a 140 (not advisable) will be very close indeed if not touching with a 630 chain.
What number of teeth front and rear, and how many links in the chain?
AL