I cannot see an obvious problem but I would personally do my code as follows...
PrintDocument printDoc1 = new PrintDocument();
comboPaperSource.DisplayMember = "SourceName";
foreach(PaperSource ps in print.PrinterSettings.PaperSources)
comboPaperSource.Items.Add(ps.SourceName);
Then pass string value to BarcodePrinter class:
BarcodePrinter tmpbcp = new BarcodePrinter(txtBarcodeData.Text, comboPaperSource.Text);
Then set paper source:
PrintDocument tmpprndoc = new PrintDocument();
tmpprndoc.DefaultPageSettings.PaperSource.SourceName = _selectedPaperSource;
perhaps your issue is something to do with the paper source index, or maybe something complete different. You should do some debugging, use a Message box to show PaperSource.Source name that you pass to the BarcodePrinter class and also the one that is used after you assign it to tmpprndoc (so test tmpprndoc.DefaultPageSettings.PaperSource.SourceName before you print)
hope that helps in some way