Dayton is a private-label name W.W. Grainger uses, or used to use for a lot of stuff. Grainger actually makes almost nothing. There are, or have been a number of Japanese trimmers sold through Grainger, like Ryobi, Mitsubishi (corresponds to Robin, I believe) and others.
Right now, I believe they sell trimmers made by Poulan and Tanaka, and they have had a relationship with Poulan/Weedeater for years, and a few of Poulan/W.E.'s older products were Japanese as well. My first choice for service parts would be a Grainger store if one is nearby; failing that, many small engine shops that service a wide array of equipment might be able to help you out.
Be aware that many common service items like spark plugs, fuel lines, universal string heads, etc. are widely available for many many brands, and if you have a nearby small engine shop, they will likely be able to help you, or put you onto someone who can.
