Do it yourself- a mapp gas torch from your home improvement store, some 56% silver, a little sandpaper, some sta-silv flux and some braze-on bits from Nova. Easy-peasy. Probably the whole mess will cost less than finding a framebuilder, driving the frame out to him, picking it up, and you'll have plenty left over for other projects. For small quantities of silver and sta-silv, your local jeweler's supply store can help. Get cadmium-free "extra easy" silver in 20 gauge. Get the surfaces to be joined very clean, fix the parts in position and slather on the flux. Heat until the flux flows glassy all around the joint. Just as the steel gets barely reddish, touch the silver to the joint. More heat on where you want the silver to flow to, but not hot enough to get actually red. Let cool some and wash off the flux with water. Best to practice on a few joints with some scraps before committing to your good frame.
While obviously not intended, I think two of the phrases above might imply to some that the flux is not in the joint itself, which it obviously is, you need some around also.
When it comes to fixing the parts together, less is often more. Sometimes the parts will stay together under their own weight. Another method is to get a bar and balance one end on the piece to be attached, and the other on any other thing at the same height, like a stack of bricks, or a jack. I get the flux from welding stores, and always check ebay for the wire.
Yes, you're correct- I should have said " slather the flux on the surfaces to be joined and the area around, then fix in position enough so that the torch won't blow the braze-on bits out of position". The parts just need to be stable- best if the gap between the bit and the bike frame tube is as small as possible so the silver can flow between easily by capillary action- silver doesn't fill gaps very well, but it will suck right into a clean well-fluxed joint.