It kinda depends where your leak is coming from. If the water is building up in the plastic trim piece and then pouring on your lap when you hit the brakes then the seal in question isnt actually the window seal and is in fact the sealing point between the metal portion of your roof and the rain gutter. the SOGI site
(Tech section) has a write-up on how to "try" to fix that. I personally never had any luck making that work. If your leak is between the side window and the rain gutter seal I strongly advise getting the new seals.
If the leak is coming from the front seal you could do what I did for the first winter I had a targa. I went and bought some butyl rubber sealer from an auto body supply house. Its a very soft and sticky rubber that never hardens. It usually comes on a roll. Simply cut a small piece, roll it out flat and lay it on the front seal where you think the leak is coming from. Put your roof back on and tighten it up, then leave it on all winter and that should keep you dry. It will still be removable when the weather dries back up.
In the end if you want it done right and can save for new seals do it. I eventually bought new seals for mine from champion and they arent that expensive. They are in fact cheaper than replacing moldy carpet and rusted floor pans.
