Can you take a day trip to Axminster? Or Martock? The staff in either place (Axminster or Yandles) will be very helpful.
I think the deeper ones are known as dowel cutters. You will need a good pillar drill, and I personally wouldn't go for a cheap cutter because machining tolerances will have a huge effect on how well it works (and the good ones are expensive).
Plugs are cut cross-grain, so you can match and hide screw holes, usually. Dowels go with the grain for strength, and a plug cutter (or even a 'dowel cutter') probably isn't the best way to do it.
Traditionally there's the dowel plate - they work surprisingly well and if you have a metalworkers nearby, they're not hard to make - they need accuracy and good steel though, Many people fit them into the bench itself, but you don't have to if they're well supported. You start with square stock ripped roughly to size. Sharpen the end, like a pencil, offer it to the hole and thump it through with a hammer.
Steve Maskery's got a YouTube demo (somewhere!) of his router table dowel cutter. Again he starts with square stock, and passes it, rotating axially, past a simple traight cutter in a jig. IIRC, he spins the stock with a cordless drill.
Found it!:
http://youtu.be/p4D8lW8uouw
I like the idea - safe, pretty simple, and gives you a decent length of dowel.
Obviously it's no good if you're actually plug cutting, but if so, why do you want such deep plugs?
Forgot something else: Appleby Woodturnings (have a web site and trade on eBay too). Their main business used to be plugs, although they've diversified into tooling recently. Would it be easier to buy them ready-made? i haven't looked recently, but they used to stock a longer type, suitable for pocket hole covering-up, in a range of common joinery woods, too.
I buy my planer knives from Appleby - I don't buy lots, but they've been easy to deal with and the prices are good.
HTH,
E.
PS: There are also the dowel cutters that are similar to chairmakers' round tenon cutters. and screw boxes (for making screws from wood). They work like a slowed-down version of Steve's router jig (or vice versa) - a blade in a wooden box and you rotate the stock past it by hand (or it round the stock!). Again suitable for dowel-making but probably not plugs. Bristol Design have them secondhand sometimes (not cheap though). Ask over in Hand Tools, too.
