[RickDeckard]

In the 5e CRB the availability roll when trying to obtain new gear is explained as an opposed roll, but I'm sure I've seen it done as an extended test using the avl rating as threshold. Did I just imagine that, or is it a house rule I've seen somewhere?

[Xenon]

SR420A p. 312 The Availability Test
If a character wishes to purchase a controlled item on the black market, make a Negotiation + Charisma Extended Test with a threshold equal to the Availability and an interval based on the item’s cost (see the Availability Interval table, below).

[Stainless Steel Devil Rat]

I wouldn't be surprised if there were a 5e example here or there when an Availability test was said to be extended rather than opposed.  Could have happened for any number of reasons, but yes you're "supposed" to roll opposed.  Subject to the Rule Zero trump, as always.

[RickDeckard]

SR420A p. 312 The Availability Test
If a character wishes to purchase a controlled item on the black market, make a Negotiation + Charisma Extended Test with a threshold equal to the Availability and an interval based on the item’s cost (see the Availability Interval table, below).

Is that 4e, or what?

[Stainless Steel Devil Rat]

A sub-edition of 4e, yes.  SR 20th Anniversary Edition.   Kind of a 4e mark II.

[Xenon]

Is that 4e

Yes.

Shadowrun 4th Edition - 20th Anniversary Core Book - 20 Years of Man, Magic, Machine.