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I used to race on 700X45 sized tires, and often found myself looking for the very biggest 700c tubes I could find.

I think that now some of the 29er-sized 700c tubes would probably fill out the inside of a 45mm tire nicely without any wrinkling or buckling.
To campare the actual width of new 29er tubes, always measure the tube's width with the deflated tube folded flat, and you'll then know which brand is likely a better fit in a 700X45c tire.
A 44mm folded width should easily fit into a 700X45 tire, even knowing that most 45mm-labeled tires are not actually that big.

When it comes time to install the bigger tubes, have some air in the tube, but not to the point of stretching it. You can later release a bit of air as you are prying on the tire, if needed, but the air volume will help keep the tube free of wrinkles during fitment.
The additional advantage of larger-sized inner tubes is that punctures lose air much more slowly, since the hole isn't stretched open. You might even be able to ride home before fixing a small puncture.