Realistically, a summer apprenticeship is like, asking for a summer long free tutorial. There is very little in it for craftsmen to offer apprenticeships, and nothing for them if they are short. Some do do it just because they want to, but it rarely pays. The old system was like the army, they teach you a skill, you peel potatoes and paint rocks for a few years in return (or fight in Iraq). The key thing is it's a binding contract wherein once you learn all you can you stay and give back labour for the opportunity you received. While most modern "apprentices" stay just till the point they realize things are starting to get repetitive, and they leave. That's what one does for a job, but it's a looser for those offering apprenticeships. They incur all kinds of costs setting you up as an employee teaching you, and modestly paying you, when the payback is way beyond the time line most apprentices have in mind.

I know a number of people who did serve apprenticeships, and they either depended on a personal network, or were personally able to influence/appeal to the craftsman. Either the person had extraordinary appeal, or they just hung around a shop long enough, refused to leave, cried, whatever it took to get in the front door. Or they married the craftsperson.

I'm not trying to be discouraging, just to point out the economic realities in most cases so you can avoid bowing your first opportunity by sounding like everyone else who has already asked. Maybe you will succeed in finding a way into the building and hanging around long enough to learn what you need to know.

The good news is that there are faster ways to learn. Get the Paternek videos for the style of building you are most interested in. That will really give you everything you need. Search out basic courses on welding, machine shop. Or get videos on those also. Hang out on the net. Get a job in a local metal fab shop. Save up money for tools. If you want to build frames it has never been easier to find out the info than it is today, the only downside to that fact is that a person who can't seem to figure that out is registering another black mark against their name when they go looking for a freebie. What you do need to do if you want to sell frames is get some experience in the business of selling frames, make some contacts, make an impression through local racing, clubs, whatever, etc... people have to know you exist and you have to know how to cost your services.