GCs have a range of mainstream seeds. There are suppliers with an huge collection of things you've never heard of. Depends what you want.
I only get them from the garden centre when they are rock bottom price, I have got some from ebay and have had good success, but watch out for postage charges, can be dearer than garden centres. My daughter has a Works shop where she lives in St Austell, they have them for 25p per packet.
I have scrounged loads from my daughter, and collected from my own plants, and now Amateur Garden mag has started free seeds right through the year, so have lots there as well.
(I know what you mean about the i pad!!!)
And don't forget the seed swap on this site
I used the GC for the basics and get the rest from some sites I have found and been suggested to me. I use Nickys Nursery. Shes good and not expensive. I dont mind spending a little extra for seeds that are hard to come by and that they are from a good source.
I can recommend two internet-based sources: Moreveg and SimplySeed. Both have a good range of seeds at very reasonable prices, and modest postal charges.
Just been caught again with the buy one get one free and free post beastie again...
Lidl always have good bargain seeds and they are very good.
Lidl seeds 39p a packet and very good