I sometimes look for a 990/10 or /12 on eBay but I haven't seen any in years. I think bitsavers.org has some software for the 990 series, like DX-10 and some language systems.
I found one in a thrift shop in the early 90s, with no storage or other peripherals. Never got it to do anything by itself, but 17-year-old me successfully soldered the (white ceramic and gold) CPU into the TI-99/4a my dad bought in about 1981. I passed that computer on to a younger kid who learned to code assembly on it.
I sometimes look for a 990/10 or /12 on eBay but I haven't seen any in years. I think bitsavers.org has some software for the 990 series, like DX-10 and some language systems.