If your buying in the USA I would look at offers from gymcor.com, I have had clients that have purchased some great machines at great prices and speak very highly of the service they have recieved.

If your located in Europe I would start with www.healthclubsathome.com, who also have a good selection of treadmill, cross trainers, bikes etc and are located in the UK but deliver throughout Europe, yet again lots of good feed back from previous clients. If your after rowing machines, new and second hand, like Fluid Rowers, Concept 2 rowing machines then www.healthclubsathome.com have a massive selection.

GOOGLE BLOGSPOT http://secondhandgymequipment.blogspot.com/

Need a complete gym set up http://www.gymkituk.com

If your looking for some good guides on Rowing Machines check out http://www.rowingmachinereview.co.uk