Although recent FLIR carrying 109s have tended to carry the FLIR ventrally it seems to reflect the size of the FLIR - it eases the CoG with 'heavy' units like the Wescam 15 and the FSI Ultramedia. These tend to be European based [still in Dyfed] Italy, Slovenia and China. I guess that such a fit would be difficult to retrofit ... lots of EMC issues likely.
But in the US where smaller FLIR units like the Wescam 12 and Inframetrics derived units tend to be used on the 109 and and 119 they are fitted forward offset to port and use simple equipment and offer a relatively simple retrofit option. Look up Cal at Meeker Aviation he knows a great deal about this aspect of the market. Operators are New York, New Mexico, etc
There are lots of manufacturers out there and Wescam and FSI may be neck and neck [not quite a home run for FSI as the previous post suggests] but there are plenty of others that will give you their eye-teeth - and a potential discount if you choose them....