I got my dial pin gauges for determining L/S size from MSC direct. They're quality and came at a very good price. Gunthings sells them too I believe as a set.
As for a replacement L/S, you might try swapping with someone on the trader boards. Seems there's always someone looking to make an even trade for a different size (been there before). Otherwise, Gunthings, Gun Parts Guy (recommended), or someone at the Fal Files are your best bets. HTH.