You'll be lucky.
There are some makes available that are 'always' available for a period, may be longish or short but they there will always be a time when 'the delivery has not turned up'.
Mind you, now that the powders no longer rely on just being available from the States and folk are using a more diverse selection from France and Finland delivery should be quite reliable.
There may still be the odd glitch but should no longer have the long periods of not having the powder you use and having to decide on waiting or using another powder and all the re-zeroing and load testing associated with new powders.
Having wrote all that drivel!!, I think the most commonly available now is probably the Vit powders and with those the price is the thing that varies most from shop to shop for the same powder.
It's taken me a good few months to find a local (1 1/2 hour round trip) supplier who has a good supply of reloading stuff in. Most other places round here seem to have some in and it's very hit and miss.
Oddly, the people I'm on about don't sell things like coats, hats etc - they're pure shooting and real gunsmiths. Other places you have to fight your way through the Barbours and Berretta Scarves for the ladies before you get near anything that looks like a gun. They probably dont make much off reloading kit.
It is frustrating - I thought that once I settled on a load I'd get my max allowance in powders, but I don't really want that much stored at home. So its probably not a great idea.
One symptom of my probs is that I am now stuck with a £35 tub of Reloder 15 that I couldnt get the bullets for, I ended up with N140 but the suppliers wouldnt let me change it. Aint buying from there no more!
I've only ever reloaded with vit. I bought the n-130 for .223 as this uses less case capacity than n-135, n-140 is what I use for the deer legal centre fire calibres,but even then n-140 can be used for the .223.
The only use I can forsee for the N-135 is if I were to load up a 125gr ballistic tip in the .308 as it seems like quite a specialised powder,for lighter bullet weights.
I'm very happy with the powders,in the shops I've been in,they have said that it's no cheaper to by a 1 kilo container than it would be to buy 2 of 1lb.
If you can get the pamphlet of vit loads from your dealer for free,it's worth having,I also find it very helpful that the Sako catalogues have the details of the powder loads for their ammunition,which is very good for giving a few pointers. I suppose it must be safe,as it's factory loads?!
I have an old Vihtavouri guide which refers to the .308 cal. using the Hornady 150gr Round-nosed bullet.
It states a max. load of 46.3 grains of Viht. N140 for 2840 FPS so 15% less to start would be 39.3 grains.
As the R.N. bullet will have a greater bearing surface than your Spitzer I would expect this info. to be safe and workable.