I photograph 4x10, and 5x7 (well the holders and film are 13x18cm).
As to why, well I've battled with 4x5, being unable to compose comfortably, always forcing a composition and being dissapointed with the results. Although I know it can be done well (4x5,8x10), it just isn't for me. It is too square (although I use a rolleicord as my toy camera!)

Sourcing 4x10 isn't difficult. Cameras are made (canham, shen hao, fotoman and lotus), holders too (plastic fidelity-type in the form of canham and fotoman) and one can even buy film off the shelf (berger bpf, adox chs 100, and ilford through the ulf order).