The mysql table knownGene in the UCSC database contains the alternative transcripts:
$ mysql --user=genome --host=genome-mysql.cse.ucsc.edu -A -D hg19 \
-e 'select * from knownGene as K left join knownGenePep as P on K.name=P.name left join kgXref as X on X.kgId=K.name \G '
(...)
*************************** 99. row ***************************
name: uc010nyg.1
chrom: chr1
strand: +
txStart: 1109285
txEnd: 1133313
cdsStart: 1114595
cdsEnd: 1133227
exonCount: 16
exonStarts: 1109285,1109645,1109803,1114568,1115036,1115413,1115862,1116110,1117120,1117740,1118255,1119299,1120348,1131979,1132472,1132817,
exonEnds: 1109306,1109701,1109869,1114713,1115117,1115720,1115981,1116240,1117195,1117826,1118427,1119471,1120489,1132096,1132566,1133313,
proteinID: Q6ZVT0
alignID: uc010nyg.1
name: uc010nyg.1
seq: MDHSCTRFIHRRGPPTRTRAGFKRGKRPRIQQRPRARVSGTIPASRLHPAPASQPGPCPAPGHCPVGPAHERPMGSSQEEGLRCQPSQPDHDADGHCGPDLEGAERASATPGPPGLLNSHRPADSDDTNAAGPSAALLEGLLLGGGKPSPHSTRPGPFFYIGGSNGATIISSYCKSKGWQRIHDSRRDDYTLKWCEVKSRDSYGSFREGEQLLYQLPNNKLLTTKIGLLSTLRGRARAMSKASKVPGGVQARLEKDAAAPALEDLPWTSPGYLRPQRVLRMEEFFPETYRLDLKHEREAFFTLFDETQIWICKPTASNQGKGIFLLRNQEEVAALQAKTRSMEDDPIHHKTPFRGPQARVVQRYIQNPLLVDGRKFDVRSYLLIACTTPYMIFFGHGYARLTLSLYDPHSSDLGGHLTNQFMQKKSPLYMLLKEHTVWSMEHLNRYISDTFWKARGLAKDWVFTTLKKRMQQIMAHCFLAAKPKLDCKLGYFDLIGCDFLIDDNFKVWLLEMNSNPALHTNCEVLKEVIPGVVIETLDLVLETFRKSLRGQKMLPLLSQRRFVLLHNGEADPRPHLGGSCSLRRWPPLPTRQAKSSGPPMPHAPDQPGARRPAPPPLVPQRPRPPGPDLDSAHDGEPQAPGTEQSGTGNRHPAQEPSPGTAKEEREEPENARP
kgID: uc010nyg.1
mRNA: BC027945
spID: Q6ZVT0
spDisplayID: TTL10_HUMAN
geneSymbol: TTLL10
refseq: NM_001130045
protAcc: NP_001123517
description: tubulin tyrosine ligase-like family, member 10
*************************** 100. row ***************************
name: uc001acz.1
chrom: chr1
strand: +
txStart: 1115076
txEnd: 1121241
cdsStart: 1115433
cdsEnd: 1120522
exonCount: 9
exonStarts: 1115076,1115413,1115862,1116110,1117120,1117740,1118255,1119299,1120348,
exonEnds: 1115233,1115720,1115981,1116240,1117195,1117826,1118427,1119471,1121241,
proteinID: Q6ZVT0-3
alignID: uc001acz.1
name: uc001acz.1
seq: MGSSQEEGLRCQPSQPDHDADGHCGPDLEGAERASATPGPPGLLNSHRPADSDDTNAAGPSAALLEGLLLGGGKPSPHSTRPGPFFYIGGSNGATIISSYCKSKGWQRIHDSRRDDYTLKWCEVKSRDSYGSFREGEQLLYQLPNNKLLTTKIGLLSTLRGRARAMSKASKVPGGVQARLEKDAAAPALEDLPWTSPGYLRPQRVLRMEEFFPETYRLDLKHEREAFFTLFDETQIWICKPTASNQGKGIFLLRNQEEVAALQAKTRSMEDDPIHHKTPFRGPQARVVQRYIQNPLLVDGRKFDVRSYLLIACTTPYMIFFGHGYARLTLSLYDPHSSDLGGHLTNQFMQKKSPLYMLLKEHTVWSMEHLNRYISDTFWKARGLAKDWVFTTLKVRPLCPPVWE
kgID: uc001acz.1
mRNA: NM_153254
spID: Q6ZVT0-3
spDisplayID: Q6ZVT0-3
geneSymbol: TTLL10
refseq: NM_153254
protAcc: NP_694986
description: tubulin tyrosine ligase-like family, member 10
(...)
Do you need to do this for all organisms?