There are a couple of aspects that go along with this.
First up, tight time-tables are totally a thing in the real world, in situations where time is a factor, it can totally be a part of the adventure/mission/storyline that the team doesn't have much time for downtime and has to focus more on the mission(s). Depending on your story it can actually be really interesting to make the team really need to be careful how they spend their limited time.
If it is important to the story that the team has limited time and that they have time to do things like buying gear off the black market, there is probably a plot point you can use to aid that step in downtime without altering the mechanics of the game. If a particular type of gear is absolutely necessary for the next job, have the Johnson provide it as part of the mission prep. Or have them put in contact with someone who can sell it to them (circumvent the availability test because the Johnson already found it).
Lastly, if you've put together a series of events that are on a strict time-table, you can also consider the possibility that the team isn't the only group involved. Maybe the Johnson doesn't want to wait for one part of the mission to finish before starting the next. Maybe they are using different groups to keep different parts of the overall scheme hidden from each-other. It's always possible that a group turns down a mission, and depending on the circumstances, that might just mean that a different team takes the job instead.