https://live.blockcypher.com/btc-testnet/pushtx/
https://insight.bitpay.com/tx/send
https://blockchain.info/pushtx
https://chain.localbitcoins.com/tx/send
The fuzzer should be able to send random data, invalid characters, etc. but also fuzz particular aspects of the transaction format such as malformed P2SH and P2PKH transactions, fields such as lock time, size, # inputs, version number, vin size, etc. It should also be able to fuzz a variety of valid and invalid script formats using odd op codes, changing the order of op codes, etc.
If anyone has recommendations about how such a fuzzer should be structured, please let me know.
Finally, if you are interested in collaborating, please contact me via private message.
Thanks!
Kristov