I know this this may not be what you're looking for as its rather expensive and not done player to player (so no deals/trades/PDs exchanged), but you can redeem DTs from this list here for a level 30 techs = 8, otherwise you'll be hard pressed to find a seller for a natural lvl 30 attack tech. HOWEVER, if you're still looking for that max damage potential as minute as it is, I recommend you expand your search for lvl 29 Megid/Grants which is far more obtainable and at a far lower price and equip a Hylian Shield because it raises all techs +1 level (not going over level cap), plus it gives Grants that sweet 100% damage boost making it the best shield to to have equipped while using grants regardless of the +1 tech bonus.
Best of luck on obtaining your techs either way!