How about adopting the strategy of being able to deliver the cameras & lenses you “release.” Frustrated here in Vegas waiting on 10-20mm & 100-300mm … try to support local shop but B&H still seem to have upper hand…
This has been the case for most high-end camera products, not just Canon’s. For the past several years, if you want a new high-end camera or lens, the best thing is to either preorder it from a major shop (B&H, Adorama) within an hour or two of the announcement, or if you have a local retailer connect with them before the announcement and see if they’ll put you on their list.
I preordered the R3, 100-300/2.8 and 24-105/2.8 from B&H all within an hour of the preorder links going live (was 6a ET for the R3, midnight ET for the more recent lenses which is a better time for you). I received each of them from B&H’s first allotment.
If you think you’ll want something but aren’t sure, preorder immediately just in case. You’ll have several weeks to cancel if you change your mind.
If you decide later, there’s still hope. Set an alert with canonpricewatch. Several times in the past couple of months, one retailer or another has gotten a 100-300/2.8 in stock. They don’t last long, so if you get an alert order immediately.
You can also use a combination of google and a phone. I decided a couple of months ago that I wanted the 10-20/4 (was packing my EF 11-24/4 for a trip, and really had to shoehorn it in). Adorama was showing estimated stock in April (it’s now slipped to May). I set CPW alerts but no luck. About a month ago, I started searching google for smaller stores, and if they listed a 10-20 in stock I called them. The first two didn’t actually have them (but they’d happily have taken my preorder). The third one, a small store in the DC area, told me they actually had one. I immediately ordered it online, and had it two days later.
Or you can wait, hope and complain. I have a pretty good idea of which approach will get a lens into your hands sooner.
Good luck!