if you don't like the factory floorplates there are a few options. Para makes and sells through their websight extended floor plates. I like these. They ad one round to your high cap and they work fairly well. other options are dawsons and grahms base pads. these too are longer than stock will add 2 or more rounds to your capacity.
If you want to stay with stock length bases the only option I know of is the old metle para bases. you might be able to find some of these at a gun show. I have no Idea how well these work.
I own 2 paras and have had no troubles with either the stock bases or the factory extended bases. I have also used aredondo extended base pads but did not care for them.
A final thought try dawson precision's web sight I seem to remember them having some flat aftermarket bases for paramags.