Close up lens/filter
Raynox lenses are pretty good. The cheapo alternatives you find on fleabay are rubbish.
The Raynox DCR-250 (+8) is quite a high ratio magnification, and therefore difficult to use with long focal lengths such as the 18-200mm. The DCR-150 (+4.8) may be easier to handle with such a lens. The Raynox lenses come with a standard adapter fitting 52-67mm filter diameters.
Otherwise, you have achromat lenses like the Marumi DHG 200 (+5) or 330 (+3) that come in various filter diameters. I am using the DHG 330 in 67mm diameter on the SEL18200 with good results. I also have a DHG 200 that I use with shorter focal lengths, such as 50mm in the standard Sony 49mm diameter.
You can also find achromat close-up lenses from Canon (250D and 500D) or Konica-Minolta (CL49-200), but I cannot advise you on these.
Many cheaper close-up lenses can be found on internet, I strongly advise to avoid them. Should you look for low prices, just ensure that the lenses are at least 'achromat' ones.