I got one for $14 US, probably could have used a funnel but this was easier as you can watch the air bubbles " burp out as you go through the process. As for getting air in with the funnel...I doubt it's really an issue because when you disconnect the tubing, you'll be letting some air in anyways until you get the cap on. It was messy enough with the tubing screwed in, I'd just buy it.
For filling, there's a few ways you can do it: The squeeze bottle is a good one; a largish syringe works well (but the oil eats the rubber seal), or even a piece of acetate folded, with a bit of a point to direct oil into the hole works (sort of a flat arrow shape); pretty much anything that directs the oil into the filler hole.
