buy an azalea at Home Depot and plant it in the ground, wait ten years (or more). Probably not the advice you were looking for, but it's probably the least expensive thing to do. The vast majority of Pre bonsai azaleas in the U.S. are pretty much sourced from similar places, only some may have had some preliminary root and structural work done to them by the bonsai supplier--some of it done very badly depending on the source.
If you're after specialty-grown older azalea bonsai material, you will pay for it. Imports, as you probably know, are thousands. Even decent old domestic pre bonsai material will be in the high hundreds.
The best source for already grown out azalea material is old landscaping. If you know a professional (if you don't know one, it might be worth finding one) landscaper, you might ask if they need free help excavating old landscapes and/or have them look for older azalea plantings in their work and offer compensation...
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