My lower end unit gear casing cracked do to water being left in it over the winter, anyone know of places where i can replace it and for a decent price..

I have no idea, but some on here might if they knew the make, year, horsepower, etc. of your outboard.

Have it welded

Ouch, got to winterise that thing better. 

Sorry to hear that........I always winterize because that's what I'm afraid of.

Were you using the boat in cold weather or was this just a result of water in the lower unit oil?

The motor is a 225 efi Mariner magnum, later got in the lower unit some how with a seal busting. Not gonna be a pretty fix!

You're looking for a Sportmaster or Torquemaster LU. The Sporty is for top speeds over 80 ish. Pay attention to gear ratio as well. It's not as hard a swap as you might think. The tricky part is lining up the splines in the shaft to the receiver on flywheel. You'll want an extra hand to rotate the flywheel to get it aligned. Note the direction it spins when you start it - don't go against it. I'd say one way or the other, but I honestly can't remember off the top of my head. There's several vids out there that show replacing the impeller, and the process is essentially the same.

If you do a search, there are several, large salvage yards around. I try to avoid anything run in salt water but that can be a problem if you are not able to see if before you buy it.

When it comes to Mercury technical expertise and parts help, Scream and Fly is good source. An awful lot of those guys eat, breath and live Mercury. If it is the torque master, it will be probably be easier to find than the sports master because of lot of the guys pull the pull the torque masters off to install the sports master because of the lower drag and higher speed rating.

If it didn't distort the housing and none of the hard parts are rusted, you should be able to have it TIG welded and install new seals. If there are any questionable hard parts, it's about as cheap to go ahead and find a used one by the time you pay the parts and labor bill.

If you take it to a dealership, they could probably rebuild one for you using the internals that you already have and just put it in a new gear case.