Don’t waste your money on a slide carrier. There are no printing materials made for printing color sides now. Also, the unique chemistry these materials require is no longer made. Even if you located a small quantity of this material, it would have become unusable by now.
Some folks have gotten the idea of using a glass slide mount and putting an individual negative frame into the slide mount. It might work, but it’s a jury-rigged solution at best and spoils the convenience of keeping the negatives in their original strips for easy storage in negative pages.
If you want to print negatives, you need a negative carrier. A glass carrier is by far the best choice for making consistently well-focused images whose focus is uniform from center to corners. The Beseler 23C, 23CII, and 23CIII enlargers use the 6 x 9 cm 8074 glass carrier (plain top glass) or the 8072 carrier (anti-Newton-ring top glass). Except for the top glass and the part number on the carrier, these are identical. If you install an ANR top glass in the 8074 carrier, the part number is the only difference.
These 6 x 9 cm carriers are used for all size negatives that fit into the carrier. You might want to make paper masks for smaller negatives as I do. For medium format negatives, you can simply lay paper strips across the ends of the frame to space the top glass upward. That’s usually all it takes to eliminate Newton rings. By using a mask or end strips between the negative and the top glass, you’ll prevent the formation of Newton rings. In that case, expensive and hard-to-find ANR glass is unnecessary.
It's unlikely that Beseler is currently making slide carriers. It’s probably still trying to sell off stock that was made and packaged decades ago. So far as I recall, the last slide printing materials and chemistry was discontinued about 2000 or 2001.