I believe Raintree Nursery in Washington state carries them.

Here is a link that might be useful: Raintree Nursery

This discussion on the fruit and orchards forum might be of interest to you before you buy.

Here is a link that might be useful: Medlars

I have a medlar tree variety Nottingham. It produces a lot of fruit but seems that the seed is not viable. I wonder what you have to do to get them to germinate?
It is a beautiful tree and does not get more than about 12 foot tall quite spreading and grows a bit like a Quince.

Medlar needs to be grafted to a hawthorn. Preferably with the graft union down low and soil mounded up so that the scion roots. Medlar scion will out grow the hawthorn rootstock hence the encouraging it to root.

Good luck,
Little John

One green world in Oregon has this, as well as many other strange and wonderful things. One of the descriptions I've heard of the fruit is 'spiced apples', which, to me, seems very generous. I found it more like baby food, but to each their own.