I use the Can9000 carbon filter.

There are a few places to buy them in Ireland. I went to a regular horticultural online shop for mine.
Shop around and see what sort of a price you can get. I prefer to do my best to order within Ireland so customs don't have a chance to check it.
I think I paid about â?¬70 - 80 for the filter. Money well spent ! I tried those odorsock types too but they didn't work too well for me. Not for long anyway.
Get an extractor fan speed controller on eBay too so you can adjust the speed of the air being sucked through your filter. Carbon filters like air that is drawn slowly through it.
Be sure you invest in a good filter... Stealth growing depends on it... The scent of weed can be detected by the most noobish of noses... Be careful !

If you go through my first grow you will see a whole section on carbon filters. I think I even gave a link to it.
If you have any more questions let me know.