If you still have any, you could maybe buy a few shirts.
Yeah, they show up occasionally on eBay as collector items. We only made about 600 cartridges that cost us about $10 each. I have maybe 50 of them left in a box somewhere. Of course, making them available would kill the price.
I was pretty proud of what we did for data compaction to fit over 200 screens like you see in that listing into a 16KB cartridge. Each image was built up from graphic objects, and the budget for all the graphics was only 4KB. (Yes, 4096 bytes.) It was a different world back then.
The source code was so large that we had a to use a program to strip off the comments before we could do the assembly. It took us about a year of nights and weekends to complete the project.