I have a seen some examples of bootstrap forms that actually look like a tax/complex printed document:
I seem to not be able to find it now and was curious if anyone can point me to an example or possibly a lib which makes this possible.
Since this is in the UX forum I'm going to give you some UX feedback. :)
Side-by-side forms, particularly for complex forms, are not good from a usability perspective. Best practices for form layout is a single column form, or possibly two very clear, distinct columns.
Save yourself some work and forget about the form library or coding this up. It won't work as well as a single-column layout and it's going to be a lot of work; if you can avoid it, by all means do.
The complex forms that you reference were driven by printing constraints. Forms were designed to fit as many fields as possible per page in order to reduce paper and printing costs. In other words, it was never an optimal design for usability or complexity – it was a design choice made based on other constraints, i.e. reproduction cost.
Luke Wrobeleski has written extensively about form design and is a good resource; here's an example article: http://www.lukew.com/ff/entry.asp?832