ISO ideas for monogramming fondant with rubber stamps, etc.?

It looks like those are special stamps with a debossed design. Meaning the "stamp" is a negative and is recessed below the surface. When pressed into the fondant, the outside area is pressed flat and the fondant in the alphabet area will appear raised. Honestly, I think that would be hard to have a uniform look, but I've never worked with fondant like that.

It seems that it would be easier to find a cookie cutter in that font. Or purchase the hearts already made.

The second example looks like a regular stamp stamped into fondant and cut with a circle cutter. I think that would be a lot easier to replicate in a home setting, especially if you don't have a lot of experience with fondant. 

The first one is like an optical illusion, it looks both recessed and embossed depending on how you look at it. Assuming it's recessed like the second link a regular rubber stamp will work. I haven't tried it on fondant but have stamped play doh with a stamp and it's deep enough. My advice is to roll out fondant and stamp with a clean rubber stamp dipped in cornstarch if you're afraid of sticking, I'm not sure the cornstarch is necessary but I know I used it for rolling it out. If it leaves a mark you can use oil on a paper towel to remove the dusty marks. Then cut it out with the cookie cutter. That will cut down on any warping from the stamp pressing on the fondant. They look really cute!

There IS a Cricut for cutting fondant - but it is a royal pain in the ass to get the hang of using (I own it). It is certainly not as foolproof as the paper version is!