I do a little of all three. My LT-40 trainer was purchased from LHS #1, my Futaba 6XAs radio was purchased from LHS #2 (they had a great price) and my Thunder Tiger 46 Pro was purchased online (no LHS carries the TT).

Supplies such as glues, paints, coverings and fuel are usually bought locally. My last batch of ARFs were also bought locally because you can open the box to inspect the contents for damage before you pay for it. I'll go online to get hard to find specialty items or something occasionally which has a big sale price (got a OS engine on a "damaged" sale because someone pulled the tape off the outside of the box and tore the outside layer of the boxtop off).

I do have a hard time finding the exact size of prop I need locally at times or a particular brand of glow plug.

I fly a lot of control line which is rarely stocked with any variety locally. In fact, a lot of the mail order houses don't carry some specialty C/L stuff so I have to order from one of the many cottage industry suppliers.