Corn syrup would be the same as dextrose as far as I know. It is just a simple sugar that will ferment and provide alcohol without providing any body. Big W has it for $4 a kilo. Some brew shops have it cheaper.
You could steep about 500 grams of rolled oats for body and mouthfeel. Normally you would also steep 300-500 grams of roast barley and maybe 100-200 grams of dark crystal. But you're not going to be able to do that because they contain gluten. You could try roasting some oats or corn or something like that, but it might not turn out too good if you don't know what you're doing and it might take a few goes to get it right.
Dark candy syrup would be an option but is very expensive. You could probably easily make some by heating some sorghum extract in a pan on the stove. There are no doubt countless tutorials online for that sort of thing. Sugar caramelised in that way will give you a different flavour than using a roasted grain.
For boiling hops, recipe tips and more information on the process, I suggest looking up John Palmer's online book 'How to Brew'. It is free.
Cheers.