I don't own one of those things, but I know someone who does and I'm 99% sure they are standard 6" jointer knives.

Also: if the knives are in decent shape it'll be cheaper to sharpen them (even if you pay someone to do it) than to get new knives. Prices vary by location, but around here it's about $0.80 per inch for HSS jointer or planer knives.

 

$19 seems like a decent price, but like Jonah says, they are standard size knives. I believe I have seen then at Home Depot so you might save on shipping costs and get them quicker.

Also, the blades are supposed to be reversible. If the other side is still sharp, then you can flip them over.

 

There is nothing special about those knives. Holbren.com is where I buy mine and the quality is very good. If the dimensions for the knives can't be found in the Owners Manual you can easily just measure the ones you have and order them from Holbren or anyone else for that matter.

 

amazon has the powertec blades for $16.99 for the set of three, and
Holbren has them for $17.77.
I have not had to replace mine yet, so I don't know if one is better.
syenefarmer says holbren, so I think I'll get a set just to have on hand.
I have the ridgid JP0600 off craigslist a few years ago. All it needed was a little cleaning,
and has worked great.

 

http://lumberjocks.com/topics/83810

have been happy with knives from holbren. There are a couple other suggestions on this thread, I was asking the same question.

 

Rigid is sold by Home Depot. I have do bought planner knives there. My guess is they would have them. Try online first at Home Depot.