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Almost any general-purpose medium power NPN and PNP transistors could be used. Try a parametric search on any major electronics stockist's web-site (e.g. Farnell, Mouser, Digikey.....).
BTW, ultrasonic generators alleged to repel mosquitos are just as likely to attract them or other pests .
Read about ultrasonic mosquito repellers in Google. Many articles and tests show that they do not work!.
My city government got rid of mosquitoes in my city (and the bats that were here eating them) by putting larvacide (safe to birds and animals) in ponds and drains.