Travel grant for summer conference between PhD and first job?
Here is a travel grant administered by the Association for Women in Mathematics: https://sites.google.com/site/awmmath/programs/travel-grants/mathematics-travel-grants

Here's one by the Society for Industrial and Applied Mathematics: http://www.siam.org/prizes/sponsored/postdoctravel.php (this requires affiliation with a US university; while your current graduate school and/or future postdoc employer might be unwilling to provide funding they are likely to be more flexible with `affiliation')

I've also heard rumors that a postdoc employer starting in August/September might be willing to support travel in the summer before the appointment starts, but this might just be wishful thinking on my part (but it can't hurt to ask?)