You don't need a deep Tuttle, a standard Tuttle will fit fine. I think the deep Tuttle are made in 46cm + fins. If your worried about the gap you can use a filler to keep the water out.
Jas,
The current iSonic 107 does use a deep Tuttle box, but as Waiting says, you can use a standard Tuttle fin in its place. Note that you'll have to use LONG bolts to secure the fin in the box.
Here is a link to fins that might interest you: select the fin you want, in the size you want, then click on the fin and it will show you whether the fin is available in standard or deep Tuttle configuration (the V Max Slam looks interesting).http://www.sunshinecoastsailboards.com.au/Components/Board-Components/Fins/Select/2012-Select-Series.html
Thanks to Sunshine Coast Sailboards for having such a good website.
Jas,
You might also hunt around these sites:
www.vectorfins.com/
www.tectonicsmaui.com/
If I read both sites correctly, you can order a 38cm fin in a deep Tuttle box. Make sure you know the thread type (metric or imperial) that you are using on the fin bolts.