I bought full size pelham roundingas as i wanted leather. They were £6 and I now have one on each saddle. You could try ebay Ingatestone saddelery have them in but I found them epensive about £20 and they were the same as the pelham roundings. Only thing wth pelham roundings you needd to measure the D ring distance as it might not fit if they are too wide. Libbys do a material one on there website,
You can get them from Frogpool Manor Saddlery www.frogpool.com for about £8 - I went for clips rather than buckles but they do both and in all tack colours