We carry standard rollers, replacement track sections, and the brackets and lag hardware that match 1980s-through-2000s residential doors, which covers the bulk of what's on the road out here. Nylon rollers in the common sizes are truck stock, as are steel rollers for older or heavier doors.
What we don't carry on the truck is anything specific to a less common door brand or an odd-width track - some of those parts have longer lead times and we'll tell you straight if your job is one of them rather than guessing. If a track needs full replacement because of a severe bend, that's sometimes a next-day parts order rather than same-visit, and we'd rather say that up front than overpromise and reschedule on you.