The values in parametric search may be wrong in two ways. Finding a part that matches requirements, but in reality it doesn’t, is one issue. The other one is
not finding a part: e.g. a 47µF capacitor may be listed with the capacity of “47” and “47000”. Even worse if some of the options for the parametric search are ranges. You need a boost voltage regulator for 24V and select the “24V” option? Pay attention: there may be options for “18–28V” and other ranges. This is highly dependent on quality of implementation of the website, but may be a trap.
As a hobbyist one may pay more attention to the “In stock” option (if available). When a given component is unavailable, the supplier may send the order in parts. Until now the companies, which I used, were either covering the shipping expenses for additional parcels or waiting until they can complete the order, but this is not always the case.
After finding a part, check local companies that do not have parametric search. Large suppliers are great for big orders, but for prototyping and hobby orders it may make much more sense to see what national-level companies and your local shops can offer. For a $4 order you don’t really want to pay $10 in shipping if you can pay $4.20 and pick the order in person.
BTW, I found out why the OP needed 256K, it's for a large number of text strings. So could easily use an external memory.
I do not know the case, but my senses are telling me that I should suggest that option: maybe that external memory could be the device with which the µC communicates through serial interface? Perhaps that device is a PC with 8GiB of RAM and terabytes of non-volatile storage? Missing the most obvious solutions is not uncommon.