Yes there are a few places that sell vases, although they are a much thinner glass than you'd be used to at somewhere like Homesense or Pier 1. They would work just fine though for the purpose! Walmart has a handful of vases but not a huge selection. I would recommend the fabric/craft type store that's across just the street from the Mega to the left called Modelas. They have a variety of vases there for about 15-20 pesos each.
For fresh flowers I'll copy and paste what I wrote in another thread:
One place I've consistently seen flowers available is Walmart at 30th Ave & 12th St. I took a picture of what the roses cost and as of last week it was 47 pesos for 6 roses ($4) and 85 pesos for 12 roses ($7). The selection is limited though so it depends what you are looking for and how much.
There are also quite a few little flower shops in town, or "florerias" as they are called down here. I went to a flower shop last night and their prices were a bit higher than Walmart although the quality is probably higher too. You can also pre-order with them via email with the shop I went into. It is called Floreria Sac-Nah and it is on the corner of 30th Ave and Calle 16, right across the street from the Walmart. Their email address is email@example.com. They don't speak English so I reccomend using Google translate if you want to email them to pre order your flowers. I would say you're having a gathering of friends and don't need them arranged (they charge more for arranging), and don't mention they are for a wedding!! I wouldn't do it too far in advance either since "gatherings of friends" aren't usually planned months ahead of time.
I took a photo of their price list (all in Spanish and pesos but I translated it) and here it is:
Carnation (Clavel): 12 for 60 pesos or $4.70
Daisy (Margarita): 12 for 80 pesos or $6.30
Pom Pom: 12 for 80 pesos or $6.30
Gladiola: 12 for 80 pesos or $6.30
Spider (Spaider): 12 for 80 pesos or $6.30
Alstroemeria: 12 for 150 pesos or $11.76
Roses: 12 for 220 pesos or $17.24
Gerber Daisy: 12 for 220 pesos or $17.24
Cala Lilies (Alcatraz): 12 for 220 pesos or $17.24
Hibiscus (Hawaianas): 12 for 220 pesos or $17.24
Birds of Paradise (Aves de Parasio): 12 for 220 pesos or $17.24
Heliconia (similar to BoP): 12 for 220 pesos or $17.24
Iris: 1 for 25 pesos ($2), price for 12 was blocked by a shelf... why they put a shelf in front of their price list, I don't know... the rest of the prices will be per 1 stem since that's the only price I could see
They had a bunch listed that all seem to be coming up in Google as lilies...
Lily (Lili): 1 for 25 pesos ($2)
Tiger Lily (Acapulco): 1 for 45 pesos ($3.53)
White Lily (Casa Blancas): 1 for 45 pesos ($3.53)
Lily (Azucenas): 1 for 45 pesos ($3.53)
Tulipanes (Tulips): 1 for 45 pesos ($3.53)
At both places, the florerias and Walmart, the selections are not as big as you'd see at home but between Walmart and maybe one floreria you would be able to find enough for your bouquets and tables, depending on which flowers you want. The roses come in various colours too although I'm not sure exactly which colours aside from the usual red and pink. The quality here can vary, but if you order from a floreria I think you will get better quality.
Another thing to consider is if you'll have a fridge to keep your bouquets in. It is very hot and humid down here and your bouquets might wilt if you leave them out for a day or two even if they're in water. I'd recommend keeping them in a fridge in a bucket of water if you can.
I'm using a wedding florist for the flowers I can't find at the shops in town and will probably buy the other ones myself. I don't want tropical flowers so the ones my florist suggested for me are roses, freesias and lisianthus. I haven't been able to find the last two in the flower shops though, only roses. She suggested silk flowers for the ones that I want that would require her to import them from Holland. She also has all kind of tropical flowers such as lilies, cala lilies, gerber daisies, orchids, snapdragons, birds of paradise and a few others. Her prices are of course higher than the flower shops and Walmart but she will arrange them for you, deliver them and you are guaranteed good quality. Her name is Vanessa Jaimes - http://www.vanessajaimes.com
If you pick tropical flowers you'll have a better chance of finding them at a reasonable price since we are in the topics, either on your own or from a wedding florist.
Hope that helps for those of you doing DIY bouquets! Again, I think the prices at Walmart are better than the florerias but you're risking not being able to pre-order and having a limited selection.