Irons upgrade

I am currently hitting the old dci oversize + black irons. I never had them fitted I bought them used off a friend back in '99. I am trying to get an idea of which new titleist irons are most similar to these beauties. I shoot usually in the high 80's and have started to play a lot more these days. Any advice or thoughts would be greatly appreciated.
Looks like everything of the modern irons match (the lengths are 1/4" longer). I went to the club archive. The DC1 irons standard lie would actually be a good fit for someone 6' tall. The 5 iron is 62 deg with .5 deg spacing to the 9 iron and 64 deg thereafter.
I shoot the same scores. I also have a pretty streamlined bag: 1-3-7 woods, 5H, 6-PW, 54-11, 62-07 and putter. Before taking the plunge on irons (that is a $1000 investment plus the cost of a good fitting) you may want to think about what could enhance your game. There are people all over the place that are going to advise to go spend $400 on the latest driver, $300 on the latest fairway wood, $200 on a hybrid or $350 on a putter. They'll also tell you that you need to have 5 wedges in your golf bag and even fairway woods should be spaced 10 yards apart. There are people that hold on to golf clubs for 20+ years because they have a comfort zone. 1. The cheapest is a proper fitting of what you have already. Putter fitting is $20 (it has helped my 6-10 ft putts quite a bit). Lie and loft adjustments on irons and wedges are $5 per club. Adjustment to length of woods (with regrip) is $15 or so (for instance adding a shaft extension is $7.00, the labor for regripping is $2.99 and shortening is $4). Labor for a reshaft is $20. 2. If you want to try a certain type of club (such as using a 7 wood or 21H in place of a 3 iron or wedges with certain lofts and bounce angles), buy used/demo first and then trade in within 90 days for a new club (or keep it). Places like Golf Mart (Roger Dunn Golf Shops), Golfsmith or Golf Galaxy have a 90 day playability guarantee (often with full store credit), even with used clubs. 3. If you already hit certain clubs well... IF IT AIN'T BROKE DON'T FIX IT! 4. Wedge setups (these are personal). I don't know how your short game is, or what courses you play. My personal setup is PW (47), Spin-Milled C-C 54-11 (bent to 55) and 62-07. This is what works for me (30 yard gap between wedges is not that difficult to negotiate) and in a wide variety of course conditions; however, it may not work for others. Some may play well with a conventional PW 56 60 setup or even PW 53 58. Others swear by 5 wedges. 5. Wood setup - do you hit your fairway woods well or do you think you would fare better with a long hybrid?
I recently asked the Titleist professionals the same question. Because I too am playing with the same clubs, even though I am shooting just a few strokes lower than you. The reply or recomendation that I was given at the time was the Titleist AP2's. Since then Titleist has come out with a new set of AP irons.

First thing to recommend is have a fitting done. you will benefit a lot more by having proper fit clubs than anything else plus a lot of fitters usually are instructors and will throw in a tip or two if they see something you can fix that wont throw your game too far off.

So if you get fit take these into account. don't show up cold. hit a bucket or two of balls. don't worry about the thought of wasting the good swings hahaha. fitters have seen it all. they are more interested in getting you in the right club not criticizing your swing. so with that being said have a good warm up. tell the fitter what your looking for. he will probably ask you about your shot type now and what you might be looking for. just answer his questions don't try to fit your self.

just let the fitter hand you clubs and answer his questions. don't worry as much about what club he hands you just worry about how it feels and your shots look. trust your fitter if he is a titleist certified fitter he has pride in his work. 

I've had a lot of pleasurable experiences with titleist fitters. they know there job.

option b if your going to just off the shelf it i would say probably ap1's or ap2's. if you tend to spray your shots i recommend the ap1's if you have average or decent club head speed and have no problem getting the ball airborne i say ap1's 

I would go get a proper fitting. That is the best way to determine the right clubs to have in your bag that will allow you to shoot the best possible scores. Given the irons you have now and the avarage score you soot, I would look at the AP1 irons. Very solid and the most forgiving iron in the Titleist line.

Is it possible to find a new Left-handed set of DCI Oversize+ Black Triangle Irons , steel shafts at Titleist?

The oversize plus Blacks were produced from 1996 to 1999.  We no longer have any parts for those.