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Define A as an External Data Source in B, and within B you can use tables from A the same way as any internal table from B itself, by adding TOs to the Relationship Graph. Now you can create a value list based on fields in a table of A.
How you manage/use a drop-down with 2000+ part #s is another matter. If the parts are categorized, you could set up a filtered value list by defining a relationship filtered by a global field where you select the category. This would give you drop-down lists that should be better manageable. If there are still too many matching records, consider using a portal for selection.