apparently a popular BOV on the cheap is the one off a 1g Eclipse. those can be had for less than $50.
the ones you are seeing on eBay are designed to drop in the SR SMIC stock piping. that's why they are 2" ID. if you want bigger piping, just buy a BOV without the premade piping, and weld a flange on whatever piping you need.
BTW, i have a blitz BOV...it's nice. <img src="#/f/iB_html/non-cgi/emoticons/inlove.gif" border="0" valign="absmiddle" alt='

'> <img src="#/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>
Eric