Where to buy Bilbo's Last Song poem illustrated by Pauline Baynes and published as a poster in 1974?

They appear on ebay sometimes. Register it as a favourite search and you will receive an email when one is listed.

I agree with Deagol, eBay is probably the best place to find one. There was one just a few weeks ago that ended up going for a little less than $40 (£20) including shipping.

[Update - whoops, that was the US poster, with the photograph, not the Baynes artwork. The same seller had a Baynes map poster up at the same time and my memory confused the two.]