On the command line, give a try to the following successful request (or see/edit it in runscope):
$ curl -XPOST https://api.finder.healthcare.gov/v3.0/getIFPPlanQuotes -d '<?xml version="1.0" encoding="UTF-8"?><p:PlanQuoteRequest xmlns:p="http://hios.cms.org/api" xmlns:p1="http://hios.cms.org/api-types" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://hios.cms.org/api hios-api-11.0.xsd "><p:Enrollees><p1:DateOfBirth>1959-03-03</p1:DateOfBirth><p1:Gender>Male</p1:Gender><p1:TobaccoLastUsedMonths>2</p1:TobaccoLastUsedMonths><p1:Relation>SELF</p1:Relation><p1:InHouseholdIndicator>true</p1:InHouseholdIndicator></p:Enrollees><p:Enrollees><p1:DateOfBirth>1982-03-03</p1:DateOfBirth><p1:Gender>Female</p1:Gender><p1:TobaccoLastUsedMonths>2</p1:TobaccoLastUsedMonths><p1:Relation>SPOUSE</p1:Relation><p1:InHouseholdIndicator>true</p1:InHouseholdIndicator></p:Enrollees><p:Location><p1:ZipCode>22901</p1:ZipCode><p1:County><p1:FipsCode>51003</p1:FipsCode><p1:CountyName>ALBEMARLE</p1:CountyName><p1:StateCode>VA</p1:StateCode></p1:County></p:Location><p:InsuranceEffectiveDate>2014-10-01</p:InsuranceEffectiveDate><p:Market>Individual</p:Market><p:IsFilterAnalysisRequiredIndicator>false</p:IsFilterAnalysisRequiredIndicator><p:PaginationInformation><p1:PageNumber>1</p1:PageNumber><p1:PageSize>20</p1:PageSize></p:PaginationInformation><p:SortOrder><p1:SortField>BASE RATE</p1:SortField><p1:SortDirection>ASC</p1:SortDirection></p:SortOrder></p:PlanQuoteRequest>'FWIW, I was able to reproduce the 503 "File not found" by POSTing the same request but with trailing characters after the url like https://api.finder.healthcare.gov/v3.0/getIFPPlanQuotesXYZ