Shaun,
I am doing loads of research on this at the moment as I want to replace my OEM 19's as they crack.
The 19's as fitted by BMW are as follows;
Front 8Jx19 ET37
Rear 9Jx19 ET39
The CSL's in staggered fitment that are available seem to be designed for the E46 M3 and come in 8.5J ET44 Front / 9.5J ET27 Rear which will fit, but you are looking at the fronts having 13mm less strut clearance, 1mm more outer clearance, whilst the rears give 6mm more strut clearance and stick out 18mm further. This will give rubbing issues if you get near to bottoming out.
Looking around on ebay, etc, there does not seem to be any CSL replicas with the correct offset for an E90.
In the US and Italy, there are examples available with the correct offset. Have a look at ebay.it, enter BMW 19 Cerchi in the search function and you will get items available. I have known people buy wheels from Italy before via ebay with no issues and will probably go this route too... UNLESS, anybody out there finds CSL replicas, staggered, with correct offsets in the UK.