Funnel-like device for putting non-liquids into a whipped cream dispenser?
Are you sure that the problem with using a funnel isn't just the size of your specific funnel(s)?

There are plenty of wide-mouthed funnels out there that you should probably try, if you haven't already, such as this Cuisinox:



Or this Norpro:



Either of those look like they'd fit into my own iSi pretty easily, but if you're sure that none of them will work for you, then I think the next best thing would be a piping bag with no tip or coupler. If you don't have one then you can always use the "snip a corner off a ziploc bag" workaround.