rt

Search for unused memory in debugged application (zeros), if You need only small memory.
If You need moore memory use redirection script:
ask "Enter new EIP"
cmp $RESULT, 0
je cancel_pressed
mov eip, $RESULT
ret

In new eip (in found free memory) You must add code to alocating moore memory (use API used in debugged application).

plugin ollyhelper will allocate 2k from the menu, if you need more then as joe described.

or use stack..

i wont money