If you are in the UK or europe, get in touch with
Praschberger and get info on a dealer close to you who can measure you up and get a ski ordered for you.
If you are in the US then you can still get a Prasch but getting a Grove will hopefully be quicker, they both cost around the same, £2000/$4000. I would really advise going with a prasch if you can, get it with the new racing shock and you wont have to upgrade anything.
Those are really the only two sitski's worth thinking about to be honest, especially if you are L1 incomplete. The tessier is a decent ski, but the shocks are crap if you are thinking of racing and generally I dont like them at all. Mogul masters are pretty crap too, decent enough to ski on but a lousy mechanism to load on chair lifts and not the strongest.
REALLY try and get your own ski as soon as possible, dont rely on hiring one from an adaptive center at all. Most centers will want to keep their ski's for the lessons they have booked already, and it will costs you too much anyway.
Plus the sitski's most ski schools have are pretty crap if you really want to improve your skiing, they will mainly be "Mountain Man" type designs with awful dual ski's on and a huge bucket that you wont fit in properly.
The bucket (seat) is the most important thing in a sitski, it is basically you ski boot and has to fit as well as a boot would, otherwise you will move in the bucket without having an effect on the direction of the ski, which aint good.
Basically get a prasch or a grove, if you can get hold of a Kevin Bramble rig then that is a great ski too,
definitely dont get a tessier or a yeti.