It might not be the fill valve. What do you have for the supply hose from the valve? One company makes one called 'Floodsafe'...that can be 'fooled' by some fill valves into thinking there's a problem, and it shuts the water off. If that's the case, get a 'plain' hose.
ANother thing that can give problems is on an older shutoff valve at the wall that is multi-turn...if the washer degrades, it can start to break up, and block the flow. If you have a plain hose, and a multi-turn valve, shut the water off there, take the hose off, and see what happens if you open it into a bucket. If it flows well, it may be in the toilet fill valve, otherwise, you have a problem with the supply or line.
