Comboboxes are accessible by default with all modern screen readers on the web. Dropdowns are a bit tricky, I hit enter on the link and if when I arrow down I don't see new options I skip to the bottom of the page since that's where a lot of the options appear. For auto complete I type as much of the value as I know ina nd hit tab. I then look to see if the correct value was filled in and if it wasn't look at the options that appear and hit enter on the proper one.