MAME source is honestly a bit hard to parse sometimes...
As a generic way to find out what you want - press F4 in MAME to go to the in-game graphics viewer - find the sprites and note the 'region'. That maps to the table describing the graphics layout for each region:
static GFXDECODE_START( blstroid )
GFXDECODE_ENTRY( "gfx1", 0, pflayout, 256, 16 )
GFXDECODE_ENTRY( "gfx2", 0, molayout, 0, 16 )
GFXDECODE_END
In this case you would have noticed the sprites are in gfx2. So then just look at the rom defs for gfx 2. (molayout stands for motion object layout in this case anyway).
ROM_REGION( 0x100000, "gfx2", 0 )
ROM_LOAD( "136057-1105.5m", 0x000000, 0x010000, CRC(50e0823f) SHA1(f638becad83307ed43d138d452199e4c6725512f) )
ROM_LOAD( "136057-1107.67m", 0x010000, 0x010000, CRC(729de7a9) SHA1(526b08e6d54cd0b991c4207c23119d2940a34009) )
ROM_LOAD( "136057-1109.8m", 0x020000, 0x010000, CRC(090e42ab) SHA1(903aa99e6e39407319f6e90102b24604884ee047) )
ROM_LOAD( "136057-1111.10m", 0x030000, 0x010000, CRC(1ff79e67) SHA1(12d408184f814bab411f567e8b29914a289e3fb8) )
ROM_LOAD( "136057-1113.11m", 0x040000, 0x010000, CRC(4be1d504) SHA1(f41ff2d31e2e0e5b6d89fbbf014ba767c7b9f299) )
ROM_LOAD( "136057-1115.13m", 0x050000, 0x010000, CRC(e4409310) SHA1(09180f1ab2ac8465b6641e94271c72bf566b2597) )
ROM_LOAD( "136057-1117.14m", 0x060000, 0x010000, CRC(7aaca15e) SHA1(4014d60f2b6590c96796dbb2a538f1976194f3e7) )
ROM_LOAD( "136057-1119.16m", 0x070000, 0x010000, CRC(33690379) SHA1(09ddfd18ccab1c639837171a763a981c867af0b1) )
ROM_LOAD( "136057-1106.5n", 0x080000, 0x010000, CRC(2720ee71) SHA1(ebfd58effebadab361dfb4bd77d626911da4409a) )
ROM_LOAD( "136057-1108.67n", 0x090000, 0x010000, CRC(2faecd15) SHA1(7fe9535b9bc72fd5527dbd1079f559ac16f2a31e) )
ROM_LOAD( "136057-1110.8n", 0x0a0000, 0x010000, CRC(a15e79e1) SHA1(3fc8c33f438fd304b566a62bbe0f6e17a696edbc) )
ROM_LOAD( "136057-1112.10n", 0x0b0000, 0x010000, CRC(4d5fc284) SHA1(c66f95af700828225a62f46437ca83453900f7fc) )
ROM_LOAD( "136057-1114.11n", 0x0c0000, 0x010000, CRC(a70fc6e6) SHA1(fbf469b8f5c6e69540743748ad994a6490ad7745) )
ROM_LOAD( "136057-1116.13n", 0x0d0000, 0x010000, CRC(f423b4f8) SHA1(a431686233b104074728a81cf41604deea0fbb56) )
ROM_LOAD( "136057-1118.14n", 0x0e0000, 0x010000, CRC(56fa3d16) SHA1(9d9c1fb7912774954224d8f0220047324122ab23) )
ROM_LOAD( "136057-1120.16n", 0x0f0000, 0x010000, CRC(f257f738) SHA1(a5904ec25d2190f11708c2e1e41832fd66332428) )
But you really you could just have found the sprites on the board by pulling a few ROMs and powering on and seeing what was broken
