They all have numbers like 471,451,27 in the jars
Probably Lane Limited or Altadis blend numbers. Although Cornell & Diehl, a most excellent blender, has quite a lot of bulk tobacco's with a number (
see here for instance) Most tobacconists will give them store names, but ultimately they all are the same. Meaning Cornell and Diehl 451 (Gray Ghost) will be the same in every tobacconist. A good tobacconist will have no objections to actually tell you the name of the blend and blender, which will help you to discover which ones you really like. Just as an example: McClelland has a blend with number 910 (Kentucky Cavendish), but so does Cornell and Diehl (Maple Cavendish). So it's worth asking
which 910 he has.
(sic)
The differences between the blends/numbers or the difference between bulk & tin ? If the first, well it will depend on the blend of which tobaccos and which blender. The differences are just legio in flavor, strength, aromatics, etc. Between bulk and tin, some swear by tin, some swear by bulk. I'd say experiment and see where you land. Personally I have both because there are certain blends I like that come in tin only.
and where is a good online store to purchase tins?
Philip and the other gave you a good list to start with there.
Good luck !