PCB - single battery design
Observations:


Power source / power requirement:

If the device operates on 2 9 Volt batteries, then the power requirement is probably not very high: Typical 9 Volt batteries work best at low current anyway. Thus, switching to AA cells or even AAA cells is an option.
In addition, lithium AA cells are relatively less expensive than their 9 Volt counterparts, with more energy per weight than Zinc cells.
This leads to a suggestion to use just 2 or 4 AA cells, preferably Lithium chemistry, and a boost regulator, to obtain the voltages actually required in the device.
Rail voltage reduction:

It is worth examining whether +/- 9 Volt rails are really required.
In the input stage of the design, signal voltage is pretty low as indicated in the schematic, so signal clipping to supply rails is probably not an issue.
For subsequent amplification / filter stages, if the op-amps are selected to be single-supply, rail-to-rail input/output (RRIO) type, this eliminates the need for a dual supply rail.
At the same time these op-amps may permit reducing the supply rail voltage, due to lower head-room needed by the op-amps, while retaining output signal voltage traversal as required by the ADC.
Dual rail power (+/-9 Volts):

Even if suitable single supply op-amps are not found, by using a virtual ground such as one of the several other options noted in this article, a dual-supply op-amp can be used on a single-rail design.
This simplifies the design further, as a single boost regulator, delivering say 5 Volts or whatever the ADC uses for supply, will allow the device to operate, possibly even with merely two AA Lithium cells.


Thus, the answer:


Use 2x AA Lithium cells instead of a 9 Volt battery. This could actually work even with a single AA Lithium cell, note the minimum input voltage below. 
Boost the supply using a suitable boost regulator, such as the very inexpensive and simple to use Holtek HT7750 boost (100 mA 5 Volt, supports input supply voltage down to 0.9 Volts)
Eliminate the dual rail requirement using a virtual ground circuit or a rail splitter IC (TLE2426)
Eliminate op-amp headroom using an RRIO single-supply op-amp such as AD8630 (Zero-Drift, Single-Supply, Rail-to-Rail Input/Output, auto-zeroing, ultra low noise, quad op amp)