The hardware store O-ring is not the same as the Remington unit is. The factory O-ring as of the last 10 years or so is metal impregnated, and silver in color when new. The hardware store O-ring will work just fine and at less than a buck vs. about $5 it is a no-brainer. The remington unit seems to last a bit longer, but only maybe 10-20% than the typical nitrile rubber O-ring. Take the original to the hardware store now while it is in good shape and match it up to one there, buy several. If you can get the Viton high pressure O-rings (typically yellow, and slightly stiffer) they hold up a lot better than even the factory unit. Hope this helps.
PS, you can slip one inside the forend before you put it on to carry a spare at all times. It will get dirty but it will not hurt it.