There are several FLV encoders available
I use: Flash CS3 Encoder ( comes with Flash CS3 authoring ),
also when editing video in Adobe Premier you can choose the
exporting formats and one of them is FLV. There are also none Adobe
tools on the market I sometimes use a Flash Video MX
http://www.flash-video-mx.com/video_to_flash_web/ which gives good results.
You can download the Adobe Flash CS3 authoring & encoder
trial and see if it works for you.