You can buy selium from a health food store (maybe even regular grocery stores) it is very high in fibre but is not that much courser than say flour (it's hard to describe) & can be mixed with water, orange juice, any foods, or soups etc & drunk, eaten & it, well I guess it sweels in your stomache & becomes fibre.

If you have trouble finding it let me know & i'll see if it might be called something else there.

You might also check throat cancer forums elsewhere which discuss recipes etc like the scd forums(irritable bowel syndrome)sites etc for recipes that you may be able to eat which would extend your variety & also ask them if they've had a problem with flatulence & what if anything has worked for them.

Also I would ask every health food person etc that I came across, whether they had any suggestions - but - before you buy anything check it out on the net to see if what you've been told seems to be supported rather than wasting tonnes of $$$$.