As for where to get earplugs… it depends on how many you want.
If you’re just toying with ’em, you can pick up at a drug store, hardware store, big-box hardware store, and probably other places in small, overpriced quantities.
If you’ve found a style you like, you can get a box of 50 or 100 pairs or more from an industrial supplier, like Granger.
There are lots of earplugs out there. Just like gear, there’s no “best” earplug. They come in different materials and firmness. One example is are the very square, firm and yellow EAR earplugs. Another are the cushy green Moldex’s. Usually the advice is to use the earplug with the highest db reduction value. 31 is better than 24. It should be printed on the packaging.
There is a trade-off though. Too much reduction and it could cause a safety issue of it’s own. For example, you’re in a factory workplace, and someone is trying to shout to tell you something for your own safety and you can’t hear them.
For a bike, I’d think the low-end db earplugs would be fine. Most of the noise comes from the wind, and having them in your ears cuts that. Assuming your bike’s exhaust is stock, the max is, what, something like 82dB? Even an earplug with 20 db reduction would bring that down to an acceptable level. I’d focus more on comfort and how they fit under the helmet, than their performance. Or another factor in my case is if they are Kawi green.