are there numbers somewhere? whereabouts are they?

come on i need a replacement!!

By their model number. Will be stamped / printed on them.

Or are you looking for a model range catalogue? If so try sram.com

no need to replace a worn out one and to know what to order (like for like as the old one was fine- came with the bike in 2009)
where are the numbers?

either on the lock ring if its original :

Or on the alloy carrier on the outside:

BTW they go in descending cost:

PG990
PG980
PG970
PG950
poo poo heavy stuff….

as long as the number of sprockets is the same then you don't need to worry about the particular version – just go with the one you can afford/fancy. I've tried most of the SRAM cassettes, they all work pretty well. It's probably 9 sprockets, so anything in the PG9XX range will be fine – replace XX with the model numbers.

e.g. cheap:-

http://www.chainreactioncycles.com/Models.aspx?ModelID=5045

tart:-

http://www.chainreactioncycles.com/Models.aspx?ModelID=8333

more here:-

http://www.chainreactioncycles.com/SearchResults.aspx?Search=sram+cassette

and the model number is sometimes on the lockring – as shown in some of the CRC pictures

wouldn't worry about like for like,
for price vs weight
http://www.woollyhatshop.com/view_product.php?id=1767
popped into halfords to see what they had and to compare,
pg970 is £54, i bought mine off woollyhatshop(ebay shop) for £19,
super saving!
you'll need a new chain aswell