I don't have a side cutter - don't even know what that is!! But often I have wished for the screw brads to be thinner - to be more mail-friendly. I've also often wished that veneers (like for countertops or tabletops) would come in fun patterns or designs (like patterned paper!) instead of just boring wood patterns.
I can't think of any other unconventional stores, but also in the home improvement stores, I also like looking at their wallpaper and wallpaper borders and wondering how those designs would look on cards. And their brown (kraft-coloured?) paper, that comes in huge rolls that painters use to cover the floor... and string. I guess the paper and string would come in more handy for Christmas wrapping (another love of mine) more than for cards. And blue painters tape for masking - just stamp on the back and cut around it - it peels off paper soooo easily.