Rotary cutter replacement blades (Ersatzklingen)

In any country, replacement blades for your rotary cutter are expensive. At the same time, it's safer and more efficient to frequently replace your blades. Not only do you get a better cut, with fewer skipped areas, but you are much less likely to cut yourself when you aren't forcing a dull blade through many layers of fabric.
Most of the time I flip a blade over if it skips, and it sometimes helps, but my blades in general get extra-good treatment. When I start a big cutting project, I give a slightly used blade a run through my sharpener (50 turns on each side of the blade with each side of the sharpener), and if it starts skipping, I just toss it. Because I've just sharpened it, I know another sharpening won't do it any good.
The sharpener is not really a sharpener, you should understand. It is a circular tool, with two different weights of sandpaper. You drop the blade onto one side and twist the handle many times (I do 50). Then you flip the blade over, and repeat. Turn the sharpener over and repeat for the other weight sandpaper.
After these 200 turns, you still haven't technically sharpened the blade. All you can hope to do with sandpaper is to take major burrs and dirt off, and there's plenty of that when you've been cutting. Take a look at the sandpaper after you've finished. It's pretty dirty, so you'll want to brush it off with your finger to keep it clean.
Regardless of how well you maintain your blades, you need to replace them regularly. Here in Germany, blades are not cheap. In the US, they seem to be a bit cheaper, but I'm not thrilled by the prices there either.
The first replacement blade I ordered was 7 Euro ($9) for a single 45 mm blade. That motivated me to look more carefully for blades in the internet.
The last time I ordered 45 mm replacement blades here, I bought them from Patchwork-Oase and paid 6.72 Euro for 3 blades. It was on sale at the time, but their regular price is 7 Euro for three. Keep in mind that these are not Olfa blades, but another manufacturer. They do fit my Olfa cutter without problems.
If you're looking for replacement blades, here are some links to websites with good prices. Please note that I've only purchased from Patchwork-Oase so far.
Patchwork-Oase budget 45 mm replacement blades 3-pack about 7 Euro
Fadenversand Olfa 10-pack about 34.90 Euro
Bacuplast has blades even cheaper at 5 Euro for 3, but I can't figure out how to order from them

Country Rose Quilts has a Prym 3-pack for 11.50 Euro, which is already too high, in my opinion

Remember the quilter's saying: a sharp blade makes a better quilt! (OK, nobody but me ever said that)