Nicked blades vary for me. It usually isn't as fast as your problem. Often I think I comes from running my rotary cutter into the ruler when I first start cutting a line.
Try eBay for blades. Just keep checking the auctions. I caught one that no one was bidding on and won 20 - 60mm blades for 23.99. You have to be persistent in checking and don't get caught in a bidding war. Once anything I want goes over a price I decide, I just drop out and try another time. Sometimes people get so competitive they pay more than retail for items.