What common exchanges that have the altcoin?
L-BTC is not an altcoin, this is Bitcoin on Liquid Network chain.
You don't need to use centralized exchanges for L-BTC when you have Aqua wallet that can be used to swap between BTC, L-USDT and others.
Sideswap.io is another alternative that can be used for desktop, or tdex.network for mobile devices.
Bisq decentralized exchange supports it, and I think Bitfinex, BTSE and BTCTurk from centralized exchanages.
There is a list of Liquid partners and federation members that includes some centralized exchanges, so some of them probably support L-BTC.
List can be found on Liquid website.:
bitbank
BTCBOX
BtcTurk
Bitfinex
BitMEX
BTSE
Bitso
Bull Bitcoin
Coincheck
Coinone
Coinut
DMM Bitcoin
GMO Coin
GOPAX
Hodl Hodl
Huobi
OKCoin
Paymium
Six Digital Exchange
The Rock Trading
Unocoin
Zaif
https://blockstream.com/liquid/