I've developed a bunion, so I need to use toe spacers (Correct Toes) that make foot wider across the toes. Without this change in foot geometry I get a blister in 2 miles. For summer I've started to use vivobarefoot toes that work OK. Everything else I've tried gives me blisters -- e.g., Keen boots have a wide toe box and don't work. For winter I have several expensive boots that I like, but they're now too narrow in the toes.

So, the question is can these boots be stretched enough to be usable. Should this be done only by a cobbler or can I use boot stretchers like this:
http://www.shoeandfootcare.com/p/101-008/product-shoe-stretcher-two-way-p101-008.html

The boots are:
Heavy leather Raichle hiking boots, great for winter/snow/ice.
Warm leather insulated XC ski boots (NNN BC bindings).
Plastic mountaineering boots with insulated leather inner boot (Raichle)-- perhaps ski shops can stretch the plastic outer boot?

If it's a bad idea to stretch the leather boots myself, can anyone recommend a good cobbler in Mass or NH?

You can do a lot more custom shaping than just a stretcher can do. Both wet leather and hot plastic can be shaped a lot with a some brute force and the right anvil. Often the right anvil is just a stout steel bar that is smooth and rounded on the end. I've use the handle of a large crescent wrench in a pinch, but a straight bar doesn't fit into the toe of a boot very well.

It's probably worth a visit to the best ski boot guru in your area.

God luck

May I suggest using Dave Page to Cobbler to help you. I had similar issue years back and he did some stretching on my right leather mountaineering boot that made a world of difference. His work was done at a reasonable cost.

Since 1969 Dave Page Cobbler has earned an international reputation as the industry leader in the repair of fine sports footwear. Our staff has over 90 years experience. We combine our skills with modern Italian presses, boot repair machinery and the newest techniques to ensure that we can rebuild your footwear to nearly new condition that will meet or exceed factory specifications.

Dave Page, Cobbler
3509 Evanston Avenue North
Seattle, WA 98103

http://www.davepagecobbler.com/
1-800-252-1229
cobbler@davepagecobbler.com

If you find a really good ski boot fiter, especially one who has experience with ski mountaineering boots, they may be able to punch out the mountaineering boots for you.