Are there any USA online shops that carry gig bags suitable for a couple of low whistles?
http://www.burkewhistles.com/home.php?cat=253
Michael Burke carries a twelve-slot whistle roll - my Burke Low D has to be broken down to fit, length-wise, and only the four largest slots are big enough for that diameter of barrel. In other words, it would hold two Low Ds, plus 8 whistles. The construction is sufficiently rugged, but it doesn’t have a handle, so I end up carrying it under my arm - that’s my only complaint.
This subject comes up quite frequently. This forum provides a good search facility in the upper right of each page.
Two of my recommended providers seem to be on hiatus at the moment: Sassafras Grove and Tuffbaggs. You might as well look at their wares though. Good stuff!
http://www.angelfire.com/mo/sassafrassgrove/WhistleRoll/Gallery.html These are first rate whistle cases
and
http://www.tuffbaggs.comI have about a dozen tuffbaggs in all different sizes and configurrations, very practical and durable.
You will also find that many things can be re-purposed to carry your whistles. Drumstick bags are quite popular in this regard. I use a couple of those myself.
Needs vary, as do solutions.
Here are a couple of threads I can recall.
https://forums.chiffandfipple.com/t/whistle-case-idea/61542/1
https://forums.chiffandfipple.com/t/tin-whistle-storage/65452/1
Hope that helps some.
Feadoggie
I use a pool cue case like this one: http://cgi.ebay.com/Quality-Excalibur-Soft-Pool-Cue-Case-Billiards-EUC-/140377153570?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item20af216022
Works great, has a shoulder strap, has an external pouch (for flasks, etc), and you can use it to carry your cue to the pub when you aren’t playing!
Pat
You can find hard cue stick cases if you want more protection. And you can make a bomb proof case from large PVC pipe if you are willing to build it yourself.
The pool cue case is an awesome idea! Im going to go to a local pool store and look at some. Thank you all for the links and suggestions. 
You might want to contact Coon Hollow Canvas
http://www.coonhollowcanvas.com/index.html
Ask her about the whistle/flute/recorder case she made for Chuck Boody (or design your own). If I recall correctly I paid $85 for a case that holds D, C, Lo A, and Low D whistles (the latter a Copeland stored in two pieces) in one side and either a soprano recorder and alto recorder (in 2 pieces) or a flute (in 3 pieces) and a soprano recorder (or more whistles) in the other side. There’s a nice piece of fabric covered foam between the sides, the outside is well padded, there’s a nice shoulder strap and the whole thing is very nice and convenient in size. I’m sure Helen could make a case to pretty much match your needs very reasonably. I’ve no commercial connection with Coon Hollow, but I have bought 6 cases of various types from her and have always been extremely well satisfied with the quality and the speed of response.
A couple of us use these at our session:
http://irishmusicalinstruments.com/shop/product_info.php?cPath=28&products_id=261&osCsid=4c1922418431ceb172727fc51e09e565
As it mentions there it was designed to carry a flute box but it opens out fully and inside there’s 6 separate slots for whistles, the slots widen enough to fit 2 whistles in each if you don’t mind throwing them in together. And theres also a large external pocket. Theres a handle and a removable shoulder strap. Height inside is around 17" or 43 cm and will hold whistles of up to that length.
I know their website is a bit tacky but they’re a great crowd to deal with, close to where I live. I’m sure shipping to the USA would be pretty cheap.