If the cross section ("profile" in weldment-speak) you need isn't in the default install of SW, you need to create it yourself.
Look up in help "weldment profiles" (basically a sketch of the cross section with placement points) to see how to create your own profile and how to add it so you can find it in SW.
There's a category filing structure as well that you have to follow, nothing strenuous, but just typing.
EDIT: you can also create this using sheet metal tools, but then you can't use it w/weldments (I don't believe).
Also, if this a chunk of unistrut, then search the ether for unistrut profiles.
If there is a profile close to the required one, you can copy it and edit it to suit, again research weldment profiles.
Make your personal library for this kinds of problems, It's very simple and save your new profile like *.SLDLFP with some information....if you send me one draft with each dimension as you need, I will make it one for you and then after you can make it all the complete library by you edit and save as and that's all.
Kevin is right that Unistrut has a downloadable library of weldment profiles on their website. At least they used too, I'll admit that I haven't looked for a while.
As Ruben says, making your own library is quite easy, and probably your best bet. I'm currently accumulating common lumber profiles. Someone has probably already made a library that's available to download, but it was faster to just start making exactly what I needed than to start Googling around (we all know what a rabbit hole that can be).
Here is a tip on the profile. Model the profile with sharps if your profile quantity is large. Model the true shape of your profile using construction lines into the same weldment profile sketch. If you have both profile present you can simply select all sketch elements. Then use the command to switch between construction & normal geometry and your profile will update to the real shape. That simple.
Additionally you can convert the construction entity geometry in any view to show the real shape.
Thank you all so much - I'm working on reading and trying all of your suggestions! I'll let you all know as soon as I can how it goes