If you sleep with your partner on a king-sized mattress, then there is no real problem. For North American sizes, the king-sized bed is 76-inches wide. A single/twin mattress foam topper would be around 39-inches to 40-inches wide, which is just one to two inches over the halfway mark. As long as your partner is not a stickler for keeping the two halves of your bed separate, everything should be fine.
But let’s say your other half is fussy about such things. What can you do? One option is to pick Australian, UK or Irish brands. Their (memory) foam toppers are 36-inches to 37-inches wide; you are shortchanged a bit, but your partner has no reason to complain.
What if you cannot find a memory foam pad of the right size? For example, let’s say you and your other half sleep on a queen-sized bed. In this case, I would suggest an egg crate foam pad instead. Find one which is not covered by fabric. This allows you to cut it down to the right size, i.e. to fit your half of the bed.
In the old days, there was a company selling “raw” slabs of memory foam. They had a few thicknesses available, and cut these pads down to the right size (width and length) for customers. Unfortunately, I cannot find them anymore. There were also companies which sold pre-sized memory foam pads without fabric coverings. These allowed you to cut the foam down to the correct size. However, the cutting process requires some effort on your part. Sadly, this choice is also no longer available.
Frankly, if you need a different type of mattress than your partner, a better long-term solution is to get two separate double/full beds with mattresses that are the right firmness/softness. If you push the two beds together, like in some hotel rooms, you can both still sleep together.