What you are talking about are the plain steel strings I normally use for the middle A and the high D string. Cello strings are available for both strings. A cello A string is very common and easy to get and there are some people who like to use them as they find the transition from one to string to another easier. The high string is another kettle of fish. There is a high E string available for cello and it is wound which makes for a more homogenous sound from one string to another.The high E string has two drawbacks. First of all, they are expensive and only available from a few places. The second and more important one is the winding. Because it is so thin, you wear through it rather quickly. This doesn't happen if you don't have frets and many folks with fretless bowed dulcimers do use them. Most folks who have tried the high E string have gone back to plain steel because of how quickly you wear through that thin winding.
I like the plain steel strings because I find that it gives me more tonal choices and they respond so much better to vibrato. They are responsive and easy to bow once you have gotten the hang of using the bow and your hand begins to compensate for the difference in mass of the strings. The other aspect that I like is the longevity. I change strings very rarely and have not had any problems either with pitch or tone. I hope others will chime in with their experience.