I don't think there has been a lull in what's available at all. Seems like there are new hot wheels a couple times per year on just the 69 and now the 67, which is about the rate it has been since the first red one. They also started using the 72 style body on the monster truck line. I found a small one a while back and picked up this one at the local walmart last week, after first seeing it in Phoenix a couple weeks ago. Green Light keeps putting different ones out. I think Johnny Lightning is back with another 69 blazer.
I'd love to see them do an original USA-1 monster truck with this body.
And I did find the 100 year 69 chevy at the local walmart a few weeks ago. I didn't buy them all. Left some for others.
I haven't been able to find the 100 year 67 though. I did see some 62's but I don't collect anything but the 67-72 style. Not enough room or money!