Search for "LED circular tube". You're probably looking for the 8 inch ones, for those that aren't advertised in metric.
You can find LED direct replacements, simply plug and play using your existing ballast. Old ballast must work, of course.
Or you can get the same thing with a ballast replacement power supply (wiring required). You'll have to find a place to put that, should it turn out to be bigger than what it's replacing. Usually a bit cheaper than the above.
There are also flat LED ring PCBs that do pretty much what you're thinking of. Most of the ones I've seen are for ceiling lights that assume they can put the power supply in the middle, so that might be an issue. Buying a cheap round LED ceiling light and removing the guts may be a fast way to get one of these.
And, there's DIY. If the project is the real goal here, go for it! Many different ways to do it. Have fun! The numbers you need to compare with the old bulb isn't watts, it's lumens. The LED replacement will consume fewer watts for the same lumen output. Color temperature is another thing you need to decide on. You can pick something specific, or you can mix in several different types of LED's to get a wider spectrum.