There are plenty of cosplay events each month (at least in Tokyo). I forgot the name of the site which list a bunch of them, but I'm sure some other anon can remember and link it.
Apart from actual cosplay events, you are usually allowed to cosplay in most doujinshi-events. There are plenty of those as well, I'd say several each month. Obviously you're gonna have to do a bit of thinking on which events you're actually going to attend. Most events usually have entrance fee of 1500-3000 a day, and cosplaying might cost extra. I'd say you're better off going to events that have doujinshi from series you're interested in, so you can buy something and cosplay at the same time. The chances of meeting like-minded people is also higher.
Circle.ms is a good site for finding events: https://portal.circle.ms/
It's meant for doujin artists and circles, but I've used it several times to find interesting events. If you see something you like you can just look up the event's own website and see if they allow cosplay. If you're into big series like Azur Lane, KanColle, Touhou etc, you can probably easily find event listed here.
Akaboo holds several larger-scale doujinshi-events a year, you can check their schedule, entry-prices and cosplay-specifications here: https://www.akaboo.jp/
They hold mostly female-oriented events (aka. lots of BL)
Studio YOU has smaller doujin-events, listed here: http://www.youyou.co.jp/index.html
Be aware tho, some of their events are really small, with around 200 circles or so (in contrast to Comiket, which has 40k circles), so if you're going alone with no friends or knowledge of Japanese you might feel lonely.
Apart from all of these, Wonder Festival Summer 2019 will be held in Chiba on 28th of July. It's a figure-event but you're allowed to cosplay there and plenty of people do. If you're in Tokyo the travel time to Chiba is very reasonable, and I'd recommend it since it's a very nice event to visit
http://wf.kaiyodo.net/