Two beds (one on adsorption, one on regeneration) will be sufficient and suitable for instrument air drying. There are a number of companies that specialize in building compressed air dryers. Compressor distributors often sell dryers as well, but do not typically design them themselves.
I would not expect you to specify bed support details to a dryer manufacturer. As TomOC mentioned, internal details are typically considered proprietary by dryer companies on standard air dryers (bottom line-you won't get a vessel fabrication drawing). Experienced dryer companies use various bed support methods.
Regeneration choices depend upon your circumstances and available utilities. Pressure swing (heatless) use ~15% of your dry air, but use no other utilities (~100 watts for the control panel). Various heated types use dry air, ambient air or recirculated air and can use electricity, steam or waste heat. Tradeoffs of capital,utilities and available purge gas will impact the decision.
CAGI (Compressed Air and Gas Institute) is the organization most dryer companies belong to...
Chuck