Putty from almost everywhere M Fent. Wilkinsons are dirt cheap, but also any of the usual DIY stores. Small tubs usually which is great cos you don't need huge quantity. Sprigs...well to be honest I've just bought very tiny panel pins, about one quarter of an inch or 5mm approx. I use my late Dad's panel pin hammer (sorry if that's not the correct joinery term, he was, I wasn't a joiner) which has a head of about two thirds of a inch wide (about 7 mm) to hammer them in.
I've bought putty in both "natural" and "brown" colours in the past. As I've discovered, brown is the best for me as it is nearer in colour to the wood, the natural needs a couple of coats of preservative to blend in. Best of luck with your purchase. If it helps, you can go to a glass supplier and ask for "horticultural glass". It's cheaper than getting "normal" window glass.