I have a pair of breast forms that were said they can be worn to show cleavage but it's not true. It doesn't look natural at all when I try to do that. It looks very obvious where it's the forms meeting my skin, and also my skintone doesn't match them at all. No amount of makeup can make it look realistic. I am however very glad I bought mine anyway though, cause they feel and look very realistic in bras which makes me feel lots better about my chest in general. They're also super easy to clean and very comfortable to wear.
They costed me 107€ on sale (120€ original price) and I wear them best in mastectomy bras. Due to their shape they don't fit well in regular bras. Their brand is Amolux, and they're made of silicone with a soft plastic film over.
However there are other ways you can create a cleavage. I would not recommend it for every day, and I'd also say to be very careful, but there is a technique you can use to basically tape your chest pushed towards the centre. And that worn together with a padded bra can give a very realistic looking cleavage. I've done it on myself, on occasion and it's uncomfortable but alright for a few hours here and there, as long as I use a tape that is not bad for skin, and not tape directly on my nipples. Also whatever you do, do NOT wrap tape all the way around your chest, cause that could dangerously restrict your breathing!
(And also for myself, because I've had a mastectomy I also have to be careful to not damage or stretch those scars on my chest, which I suppose does not apply to you.)
I've used medical tape that is commonly used for bandaid. But I've also heard that the K-tape most commonly used by ftm's for chest binding, has been used by mtf's for creating cleavage as well. I haven't tried it myself yet, but I've been meaning to, whenever I can get around to order some. It's a rather new thing to use that tape for these sort of things, but the tape itself is a sport tape.
My chest is entirely flat but I can use the small amount of fat and skin there to push it towards the centre. If the chest is not quite as flat this technique will be easier and provide a better result though. But please be careful about what type of tape you'd use. Definitely don't use duct tape or anything that can damage skin when removing it. There are vids on youtube on how to do this, but it's not difficult.
Another more safe technique is to use makeup to create the illusion of highlights and shadows on the chest, but somehow I doubt this would look realistic in an irl, non-stage environment. It's also a bit annoying to have makeup constantly smearing onto your shirt.
