MegaPrims. Help pls

Ok I know we shouldn't use them but I am in need of a 50x50x0.01 and a 50x25x0.01 megaprim to surface a lake. Can anyone help ?

Mega prims are not illegal, nor disencouraged by LL. The only instance that will prompt a Linden to show up and delete the prims is in cases where the prims are a nuisance and someone sends an AR about them. Using them to cover a lake seems like a genuine use for them. Remember to make the prim(s) phantom and non-physical, that way it'll bring you no problems.
You can get the full Mega Prims package for free in SLExchange.

Get this box:

http://slexchange.com/modules.php?name=Marketplace&file=item&ItemID=206333

That's probably the most complete set.
Don't pay anybody a single $L for huge prims.

Be very VERY careful when rezzing the really big ones on the mainland. If a huge one rezzes around you (say a 1/4 sim size one or bigger) you might not realize because they're only visible from the outside. Would cause one heck of a problem for people living around it (skewered buildings and such), many of whom would AR you.

People might AR you anyway. I got ARd last week for using huge prims as a foundation for a build on my own land. Wasn't unsightly or causing a problem - it just helped me save on prims for such a big surface - so the investigating Linden made no suggestions, told me not to worry and went on his way.

Thankyou all for the help - much appreciated

Hmmm... how to get the attention of a Linden. Rez a mega-prim.

I hope this doesn't drive the wrong sort of behavior.

The last thing I want is to cause any problems for others. In this case the parcel occupies 3/4 of the sim and I am being very careful to make sure nothing strays over the edge.
Normally I would not resort to using a megaprim and the prim count is not an issue, the need arose as I simply could not achive a 50 m square water surface (scripted to move) without getting seams showing.