check with a local plumber, since they solder copper fittings all the
time, or ask at a local welding shop, since they tend to get all sorts
of custom jobs.

heck even a roofer can probably fix it, provided they do copper roofs

its actually easy, clean both parts of all mating surfaces, apply flux,
which helps clean and prep surfaces. put pieces together clamp if
necessary. probably best to use a propane torch, heat the pieces, only
apply solder once all parts are hot, always apply solder to the parts,
DONT HEAT SOLDER WITH FLAME HEAT SURFACES, LET SOLDER MELT AND FLOW!
wipe with cloth to smooth it out

there are books about soldering

when done let cool, clean the area thoroughly and enjoy your repair!