I expect your options are:
1. Build a table that has the names of tables and queries you want to allow search on. Make this table the RowSource.
2. A list typed in the RowSource and RowSourceType set to ValueList.
3. Give tables/queries (actually, recommend use just queries so you can limit the fields available) a name with a prefix like S_. Use this prefix as criteria for the RowSource SQL that retrieves the list from the MySysObjects table.
SELECT [MsysObjects].[Name] AS ObjectName, IIf([type]=1 Or [type]=6,"Table","Query") AS ObjectType FROM MsysObjects
WHERE Left([Name],2)="S_"
ORDER BY [MsysObjects].[Name];