can someone recomend a garlic to me

Would it be a really bad idea to use shop bought garlic??

IMHO, a bad idea yes, but I will confess to having a commercial interest in garlic seed as well as final harvest.  (Not in Really Garlicky Co whom I have recommended in earlier posts)

Is it more prone to rotting or disease?

Not necessarily.  It is more a case that you don't know what you are getting.  I originally sourced 8 or more different garlics from wholesale grocers and only 2 merited replanting.

I agree with you there, WG. Much store garlic is grown overseas, often in warmer climates and different conditions. Canadian fresh garlic is primarily from China and is SO flavourless. Why grow mediocre when you can grow superb? Once you can source some good varieties, you can keep saving your own seed. I'm off to a garlic festival this coming weekend and hope to buy a number of varieties new to me and grown at local farms that I was previously unaware of. One farm calls itself Big Ass Garlic    Guess we know their mindset.

Last season, for Autumn planting, I grew Thermidrome bulbs - Organic Gardening Catalogue: pricey on postage but a reliable cropper and much stronger than supermarket varieties. This Autumn I shall replant some saved bulbs but am also trying Purple Wight from Mr. Fothergill's - free postage on onion sets, garlic and shallots last time I looked. Also worth a look is the garlic farm http://www.thegarlicfarm.co.uk/
Might look next year at Albigensian Wight.