Not so much Conversion pieces - rather complete miniatures...
Urban Mammoth and Scotia Grendel with the Void and Urban War miniatures, the civilian militia flavor scale best with
GW of the ones which are out there now (same chunky proportions):
http://www.urbanwarthegame.com/acatalog/Generic_Troops_Blisters.html
http://www.scotiagrendel.com/Products/index.php?main_page=index&cPath=59_67
Copplestone Castings Future Wars are similiar in proportions - though they tend to be slightly shorter in general (not by much though):
http://www.copplestonecastings.co.uk/range.php?range=FW
That line is also produced by a couple of other companies (EM4 has some and Mirilton
IIRC).
Hasslefree and Heresy have some nice stuff - but they are more realistically proportioned than
GW figures. If you aren't going to be mixing them up - those two are a good source as well.
http://www.hfminis.co.uk/shop?category=miniatures~adventurers
http://www.heresyminiatures.com/shop/index.php?main_page=index&cPath=2_14
RAFM also has a good selection of Civvies with guns in their Modern Day Heroes line up - they tend to be a little taller and more realistically proportioned as well though.
http://www.rafm.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=RAF&Category_Code=USXMin&view_perpage=all
Depending on your specific needs, you also have Bronze Age:
http://bronzeagemin.com/miniatures_html/32MM/SCI-FI/Mercs/32mm-Sci-Fi-ADV1.htm
They are 32
mm figures - which makes them a bit taller than the
GW figures, but they are more slender.
Of course you also have the GOT figures from Antenocitis Workshop:
http://www.antenocitisworkshop.com/antenocitis/g-o-t-figures-1.html
Again, they are taller and more slender...in line with Infinity as opposed to
GW.