How can I source a custom field from sales order if I am going to use the invoice template? Is that even possible?

Custom fields populated on Sales Orders at the time of creating the Invoice should automatically copy over to the Invoice as long as the same field is exposed on the Invoice form.

Oh, so yeah I think that fits my case. So can I just access it using record.createdfrom.custbody?

i guess what i'm saying is you don't need to access through the createdfrom record - you should just be able to say

record.custbody...

because the invoice will have it on its own