I have two items to help keep bread slices uniform for sandwiches:
One is a (plastic) cutting guide that came with an expandable bread
keeper. I got mine through King Arthur Flour... they say the
manufacturer no longer includes the guide, and they are looking for a
replacement. You might try googling, ebay, etc. The guide is a
terrific help, and the plastic bread keeper is pretty good, too!
The other is a bread knife with an adjustable guide. Mine is made by
Fiskars... I've had it for years and don't know if it is still made.
I think they are often called bread or deli slicers.
If I'm making a lot of sandwiches, I uses the plastic guide,
otherwise usually the knife.
Hope that helps!