Recently Dungeon Fantasy 8: Treasure Tables and some of the items in it came up for discussion.
Here is how my game handles this stuff:
DF1: All equipment/magic items are available unless I say otherwise. The default answer is yes.
DF6: All magic items are potentially available, but only if they appear in play or indirectly in play (via rumors, etc.) Nothing is available for purchase off-the-shelf. The default answer for availability is no.
DF8: All magic items, unusual items, etc. are potentially available, but only if they appear in play, as DF6 items. The default answer for availability is no. Some items may be available off-the-shelf, but generally they are not.
Other supplements: As DF8. Not available unless demonstrated to be available, so don't ask until you see it. This includes Low-Tech, although most TL0-4 weapons are available.
Important note: never bet your character's life, development, future tactics, or end-game plan on any equipment you have not seen used in actual play. That which isn't shown to exist is not necessarily going to come into play just because stats exist for it.
This includes stuff I have personally written - if I wrote it, and I want it in play, it will show up in play, rest assured.