Try this link at the IGI Batch number for England:
http://freepages.genealogy.rootsweb.ancestry.com/~hughwallis/IGIBatchNumbers/CountyBuckingham.htm#AIt has marriages for Amersham 1561-1812. There a number of Walker marriages here and it's free!
From my Phillimores for Amersham I have found the following marriages:
Thos. Walker & Sarah Child 23 Nov 1640
John Barns of Chesham, & Sarah Walker 2 Oct 1712
Rich. Walker & Eliz. Provis 18 Apr 1715
Thos. Walker & Hannah Hide 29 Mar 1719
John Dear of Chesham & Rebecca Walker 18 Sept 1746
John Ford of St. Peters Chalfont & Mary Walker 30 Apr 1754
Wm. Walker of Fulmer & Ann Knot 30 May 1776
Stephen Miles & Ann Walker 6 Jun 1778
Jos. Walker & Susannah Miles 17 Dec 1782
Benj. Walker & Elizabeth Butcher, lic 30 Jan 1790
There are many other Walkers as it's a very common name but the above are those specifically in my Amersham listing.
Other sites I often use include rootsweb (Worldconnect option), free and you often find a branch already researched although it pays to double check.
When my ancestry sub runs out I still use it and go to the public trees section.
Obviously there are also the FreeBMD web sites and there is also FreeCen although I have not used the latter much. It also pays to look for online parish clerks for a county.
Hope this helps
Regards
Steph