Making microtonal music with software can get pretty expensive. Fortunately, there's a large supply of free music software that might be able to fill many of your needs. Considering that you can easily spend hundreds or thousands of dollars on music software, it's well worth checking out free versions first.
There's a lot of places on the web to get free or inexpensive software, but I would recommend that you check out Hitsquad. It has a very large directory of sites that offer music software. A lot of the software isn't free, but much of it is. You can also find a lot of demo versions.
Of course, most of the software doesn't offer microtuning abilities. It can still be difficult to find what you are looking for, especially if you are into more exotic microtonal scales that require unconventional keyboard mappings. Still, it doesn't cost anything to browse the titles.
If you are thinking of spending a couple hundred dollars on some software with microtuning abilities, you will likely want to see if you can find a demo version first. It's been my experience that much of the microtonal software is so limited that it's virtually worthless for the type of music that I want to make.
You may also want to consider whether the sounds of the software match the tunings that you are interested in. It may sound like a lot of fun to use fat sounding synthesised tones with exotic and dissonant scales, but the actual result might not be pleasing to your (or any one's) ears. It can take a lot of experimenting to match sounds with scales, just another reason to try before you buy.
If there's a problem with Hitsquad, it's that it can be too much fun. It's easy to waste a lot of time downloading every title that looks interesting. I recommend that you check it out, but use some moderation so that you still have time to actually make music.