Welcome to digideep
Not sure of your budget. Ikelite and Fantasea make housings that are probably the least expensive and Fantasea, since they include a standard port, maybe the least expensive.
Next in line (dollar wise) is Aquatica
http://aquatica.ca/subpages/catalog/housings/ad40x.html
I have an Aquatica and love it, some of my general thoughts are here
http://www.digideep.com/english/info/html/aquatica-30d-housing/
Other housings are here
http://www.digideep.com/english/digital/photo/camera/Nikon/D40x/25/2888
EDIT: Ah I see the lens size in the title now. missed it before.
Aquatica does not make a port for that lens as far as I can tell. http://aquatica.ca/subpages/catalog/lens_charts/nikon_type_2.pdf
Ikelite does have an 8” port http://www.ikelite.com/web_pages/dabigadoma.html which supports the lens.
That being said, note that Aquatica does have an 8” port in the above link and the lens may work in that even though not listed specifically. Worth checking into. For the most part once you have the port, the gear rings are different sizes and the lenses work behind the port.
And some lenses, though nice, may not be what you want underwater. Overall lenses with less range, such as 10-24, 18-55 and some fixed longer range lenses (60mm, 100mm) are what people lean to due to better results. Though at some point I may try my 18-250 Tokina (which I carry around for general above water use and it does a nice job)
One day there will be a 10mm-250mm lens that works well underwater for dSLRs (well at least I can dream
)