I'm afraid I cannot answer the whole question but I can shed some light on record distribution in general and how Joy Division's albums were distributed.

Let's talk record labels first. There are two kinds of record labels: major and independent. Major labels sit under an umbrella organization that owns multiple labels. Each of these umbrella organizations own their own distribution companies. An example of this system is Sony Music Entertainment, one of the largest music company in the world. Sony owns labels such as Columbia and RCA. The records released by these labels are distributed by various distributors owned by Sony depending on which location. Distributors can be, but do not have to be, different for different markets.

Independent labels are not owned by a major recording company such as Sony. Thus, they have two main methods of distributing records: major distribution or independent distribution. In the first kind of arrangement, the independent label is the one responsible for finding the talent and recording the music. The product itself is either released under the independent label or sometimes under a label owned by the major. Either way, in this setup, the distribution is handled by the major. For example, an independent label might sign with Sony to distribute its records.

The second arrangement is that of a true independent label. These labels have no connections to a major. They are financed by the owners/investors and the sales of its records. Their records are distributed by independent distributors. These independent distributors can be other independent labels that operate a distribution network of stores in a particular region.

With all this out of the way, let's look at Joy Division's record label in 1980 - Factory Records. Factory fits into the second category of independent labels as it did not use major distributors to distribute their records. In the book Shadowplayers, James Nice writes of Joy Division's 1979 debut album Unknown Pleasures: "[i]n America the album was distributed through Rough Trade, who had recently opened a shop and office in San Francisco."

Rough Trade Records is British label and a chain of record stores, and was the distributor for Unknown Pleasures in the UK. It wasn't a major distributor by any means; Tony Wilson, co-founder of Factory, claimed there were about fifty stores in Rough Trade's UK distribution network. Early in 1980, before the scheduled Joy Division tour in May, Factory had branched out and formed Factory Benelux and Factory US to handle local markets. Rough Trade, with their new San Francisco office, was the distributor in the US.

That is how the record would have reached the US in 1980. I cannot comment on what the process would be for a record store to get the album from Rough Trade. My assumption would be that Rough Trade sent out copies of the album to all the stores in its distribution network but that is an opinion.

I hope this helped a little and I hope someone else can shed light on how a store would receive a copy.